I'm looking at installing electric underfloor heating in my new
bathroom and have a couple of questions about the floor:

1. The existing floor is tongue and groove timber over joists and it's
nice and solid (no flexing that I can notice) and it seems level...
Would there be any problems using a board such as Fastwarm screwed
directly to the existing floor, followed by the electric cables, then
a levelling compound and finally the tiles?

2. Obviously the normal recommendation is 18mm of marine ply but I am
worried that I don't have clearance for this along with everything
else (I have 40mm from the current floor to the bottom of my new glass
door!). Is it worth going 50/50 with say 5mm ply and 5mm Fastwarm or
similar?

Any advice greatly appreciated!