Hi Gibster,
Subspecies refers to the division within a species when (most often through geographical isolation) a distinct population of a species occurs which differs from type through consistent and recurring characteristics. The allocation of subspecific status is somewhat arbitrary; however, the general rule of thumb is that the population must be sufficiently geographically distinct from type. The distinction may also include such things as larval food plant preference (to the degree that the usual larval host plant of 'type' is rejected utterly, resulting in the inability for subspecies to translocate).
Where the clinal differences are insufficiently distinct to award the population(s) supspecific status, the dimorphic 'race' is known as a
form (for example
form scotica of the Mountain Ringlet).
In all likelihood, the extinct subspecies
cretaceus of the Silver Studded Blue (calcareous habitats, south east England) was nothing more than a
form rather than a distinct subspecies.
Was
masseyi a distinct subspecies? well they were certainly very different from type. Almost all of the females exhibited considerable blue scaling and both sexes but especially the female) bore longer, narrower wings than argus argus. Ssp
masseyi flew in north west England, mainly Lancashire and the former county of Westmorland. If not a subspecies,
masseyi was
certainly a very distinct geographical cline; a form.
The Silver Studded Blue is an
extremely variable butterfly, both regionally and within individual populations. I have long studied this butterfly in this respect. 'Blue females' crop up with regularity in most colonies (except as I mentioned the New Forest populations where it is rare). There is significant variation in the size of the butterflies within any population, with some males being as large as Common Blues and some females frequently being absolutely tiny. The New Forest Heathland race generally produces uniformly small specimens (by national standards) while the Cornish Dune populations produce many 'over sized' individuals, some of which are really pretty big, and quite strikingly marked. The species is generally very variable, with much variation in underside ground colour, the extent and size of spotting, the extent of the black margins to the male uppersides
etc etc. These variations occur widely within any population. Freshly emerged males ordinarily have a very silvery-whitish underside ground colour which quickly wears.
The Great Orme population are nicely distinct, both in average wing span, colouration in the female, and emergence dates. Every colony will have individuals which do not conform to 'type' for that particular colony. I could show you a series of photographs from a Dorset Heath population which are large, bright and with very blue females, but this would be the exception rather than the rule.
There are no books available which act as a catalogue or key to British Butterfly aberrations or subspecific forms, although for each species there is a finite amount of aberrations which have been described and named. The naming has been largely rational and consistent, but It really is a case of trawling through literature and old periodicals, some dating back 150 years...!
There is the on-line reference on the Natural History Museum website (the Cockayne Database) however this is very limited and only illustrates a representative handful of aberrations per species so is flimsy as a means of accurate identification. It's function is to demonstrate the sort of variation in markings that one can expect within a species, but it is not a definitive guide by any stretch of the imagination. Like many works there are also errors in the names applied to some of the aberrant specimens, but as it is not meant as an identification guide this is of no real consequence.
I would not encourage you to pin specimens for identification; photographs are more than adequate and will readily illustrate the features which betray the individual as an aberration, or any manner in which the specimen is distinct from it's fellows.
No one "bows down" to me Gibster in any way, shape or form; please do not use the expression with reference to me, it is truly awful and implies some form of hierarchy of which there is none.
I am a mere enthusiast just like you, every one has their own speciality and there are many contributors to this site who are a great deal more knowledgeable (particularly in the field of photography, of which I am generally in awe of most people here) than I.
My knowledge of British butterfly aberrations and forms comes from year upon year of fascination and study. Just like any subject, one gradually picks things up and assimilates information. I have also gathered together a whole host of dusty old periodicals and journals (much to the delight of my partner!) which enables me to have an awful lot of information to hand which would otherwise be unavailable. Years of study in the field has also enabled me to gather first hand a wealth of observation based reasearch pertaining to certain species. But this does represent many years of many lost weekends so I would hope to have something to show for it.
Yesterday I was fortunate enough to spend the majority of the day on Portland with the Silver Studded Blue population there; arriving at about 07.45 when the insects were still at roost. Hundreds were on the wing in the usual places. There is something about studying these butterflies on Portland which really appeals to me for some unfathomable reason. The melancholy atmosphere of rusting jaded industry (the sites are utterly 'brown field'), the stench of the land fills, the dog crap everywhere (something which I ordinarily loathe in the extreme - trod in two and
almost dropped my note book in one other). There must be something about watching these beautiful azure butterflies within what would otherwise be quite a depressing environment that somehow appeals to something deep within me.
If I were only to ever be allowed to see and study one butterfly species for the rest of my life it would be this one without hesitation.
Felix.
Ps. Good luck with the rest of your Grand Tour...!
. Not many butterflies were willing to play ball either, this White Admiral was the only good photo opportunity of the day. 
I'm glad I nailed it, they are a frustrating butterfly, and I doubt I'll get another opportunity like that for a while.
) Also we saw a handful of 'midget' examples at Prees Heath. The Great Orme colony we found consisted of individuals all clearly smaller than argus. Are Silver-studded Blues simply more prone to variation than the other British blues and if so, how valid are the subspecies? Which leads on to my next question...