During some refurbishment work we have removed all
the floor coverings from the kitchen, dining room and hall,
in preparation for latex self levelling (All solid floor).
The hall was covered with carpet under which were small
wooden blocks (parquet flooring). These were stuck down
with black bitumen like adhesive.

Over the weeks of refurbishment the tackiness of this black
glue has been annulled by the copious amounts of dust that
have been produced. However this dust layer has show up a
worrying patch of what appears to be damp.

The weird thing is that it's right in the middle of the floor. It has
slowly grown to about 3 inches in diameter. No sign of damp
anywhere else. There are no buried pipes in the floor.

Nice pictureof the damp patch here... :-)
http://www.gillandroy.com/diy/odds/damppatch

I have a suspicion that when the house was built in 1963 there
was no damp proof membrane laid below the solid floor.
Some years ago I dug a hole through the dining room floor
and found no membrane.

What should I do? Ignore it - it's not been a problem in the last
18 years :-) Or perhaps dig up the offending area and fill with an
SBR & concrete mixture? Or what... ?

Thanks,
Roy