Another discovery: be very careful when re-assembling and remove SD and SIM cards first. They can get caught under the housing and I have broken mine!
I've extracted the battery and am awaiting a replacement. It holds a charge OK but has bulged to 10mm thick plus some gaseous inflation. This caused the motherboard to bulge in response and popped the screen out of the housing. I suspect that it was, before the "pop" made it obvous, the cause of the failure of the torch/flash. This is because the connection is just by contact pressure between back and motherboard and in the rough centre of the device where the bulge pressure would be most noticeable. So I am hoping for a double cure...