This began as just a glimpse of stoneware in the mud; I started excavating and when I was confident it was going to be complete I took the second photo. My hunch was right - it's a complete R White's ginger beer bottle.

Further to my recent post about this heavenly fragment, see the reply below from Alastair Owens who tells us that another of these plates was found in an excavation in Limehouse! See his photo; the plate is the same, but hand-coloured, as were many of them...