About 6mm. Landed on the chopping board in the kitchen!
I was wondering about Plutella but I'm probably wildly wrong. Unidentified Micromoth by Alastair Rae, on Flickr
I dug this shot out of the recycle bin. My county recorder agrees it is probably Diamond-back Moth Plutella xylostella Record shot of unidentified moth by Alastair Rae, on Flickr