Thanks all for replies. To summarise it looks like:
1. Mixing Floors is okay
Okay to mix new solid and existing timber suspended floor.
2. Solid better insulator than timber (for hydro UFH)
To meet Building regs, and get a reasonably warm result, suspended beam and block OR ground bearing slab will be more thermally efficient (require less insulation) than suspended timber, though...
3. Timber floor faster heat-up (for hydro UFH)
"A wooden floor can take up to 30 minutes to come to temperature while a concrete floor can be hours." (http://www.bhl.co.uk/article/2562/BHL+Guide+to+Underfloor+Heating)
3. Beam & Block cheaper than Slab
(Suspended) Beam and block may well be cheaper, but
4. Slab is best "solid" option for this size job
Ground bearing slab will need less space, and be more suited to the size of job (extended area only about 12 sq. m).
5. DPM needed for Solid floors
For both suspended bean & block and ground bearing slabs, existing concrete will need to be excavated to put down a DPM.
Assuming I got the above right - raises some other questions....
a. Will mixing slab and suspended timber be okay for tiling over as one floor. I have concerns that we are more likely to see movement, at least where the old and new meet, if not only on the old timber (though old has been pretty firm, no cracks in existing tiles/grout over 20+years).
b. Will there be a marked difference in warmth of the two (under) floors - I assume the old suspended timber one will leak a lot of heat but be more responsive under foot - even if "Backer Boards" are deployed throughout.