The pure black form is ab. nigrina Weymer, 1884. Other forms, with more or less white on, have other names. Yours looks pretty good for ab. obliterae. I've never seen any of them and it gives me a real thrill to look at these pictures!
A useful site for forms and aberrations is the Cockayne site. You will find white admiral here:
Anyone visiting Fermyn Woods should keep an eye out for ab. obliterae which are regularly seen there. This pic was taken two years ago. (I may have posted it at that time - if so, apologies!)
Alex
Yes, Fermyn Woods is where the 'abberant' photo I took (in the main section about White Admirals) was taken... and there was at least one other specimen on the wing there at the same time. I think Pete told me this form may be caused by temperature fluctuations during the pupal stage. With the weird summer we have had so far, there may be a few more about this season.