When a plumber was replacing my CW cistern he noticed that the heating
feed/expansion cistern drained and reported that I have a leak in my
heating system.
He said the leak was so bad that it is definitely downstairs as
otherwise there would be obvious damage to the ceiling below.

I have looked all over for the leak, the only place left to check is
under two expensive floors (the floors are raised on joists about 50cm
above the ground).
Before I start damaging expensive floor boards, is there any other
possibility?

One thing that springs to mind is that the HW cylinder could be
leaking water internally (i.e. leaking hot water from the coil into
the tank). Does this sort of thing happen much or am I just graping at
straws?

Any advice much appreciated!
Chris.