Sadly the sauna/burnt mound in Moseley Bog is little more than a small mound by a stream littered with blackened stones. I could send you a picture if you want, but there's not much to see.
There's a pdf document from Mike Hodder about burnt mounds being used to heat water at:
I might have squeezed in there, but squeezing out? It would have been one of those Pooh bear moments I feel. Waiting for a fortnight to slim down a bit. Amazing place.