sorry my head's gone blank on this... I know I can capture 25F(PAL) via the HV10. can I then record the 25F timeline back to tape on the HV10 or would I have to use the A1/H1 as the HV10 doesn't record in 25F or doesn't it matter as the timeline is already 25F?

Your editing software will output your full frames (25f) as interlaced footage (i.e. two, in this case temporally identical fields per frame). That's also the way your cam is recording the footage and therefore outputting it via firewire.

is that how 25p on the (PAL) HV20 is recorded?

Even though nobody has seen the HV20 PAL version in action yet (some say it's coming this or next week), it will most definetely store the 25P footage in an interlaced stream. This is a consumer camcorder and 25P footage has to be compatible even for standard tv sets. There's no problem with that though, it doesn't need inverse telecine or deinterlacing.