Ahh, as I'd like to keep my existing pump etc (its working, why replace it,
and its easy enough to replace myself if necessary) am I therefore
restricting myself to not having a system boiler? Are there any modulating
non-system boilers out there?

Yes. The Ideal Icos might be ideal for you (excuse the pun). It is available
without pump and will run on a feed/expansion tank. (I wouldn't recommend
doing so, though, as sealed pressurised operation is better).

Christian.