Guh - this could have used at least another 5-10 minutes - and a couple more blasts of flash. I should've checked the histogram but was swayed by the piccy on the camera screen in the pitch black.
So, a bit of fettling later and it's maybe not so bad, provided you don't look too closely at it!

Also, my torch bulb blew on the way back! It's a good job my 'phone has a torch built in. Another shopping trip looms.