I've wired the capillary stat in now, and as predicted it seems to be doing the trick just pumping round the circuit formed by the ABV.Excellent - I'll report back once I've wired the new stat into circuit next week!
I've put the extra stat inside the boiler cabinet and brought an extra permanent live across from the main junction box to power the pump bypass. In fact I haven't actually wired it in permanently yet as it occurred to me that I now needed to isolate both my new permanent live to the boiler as well as the existing switched live. I'm using a three pole isolator switch for this (the sort that are used for timed fans) in place of the current switched, fused spur. The whole installation is already fused elsewhere in the room, so I don't think that losing this fuse local to the boiler is a problem - unless you advise otherwise! I could put a simple fused spur outlet in as well if thought to be needed.
Thanks again for all your help, and the interesting diversion about steam thermodynamics - I haven't seen some of those terms since I was studying Physics at school which was a very long time ago