Is the boiler pump inside the boiler (system boiler). Did the problem start after the installation of the extra expansion vessel? is it positioned on the flow manifold or the return manifold?.
The boiler is a Baxi Platinum boiler and has a pump inside it. There's another pump on the manifold of the UFH. The problem started after the UFH was fitted but before the expansion vessel was fitted.
Sequence of events was...
Boiler running fine with just rads.
UFH installed and turned on
Boiler dropped out with E125 (which the manual lists as "Circulation Fault (Primary Circuit)"
I powered off the UFH Wiring Centre (it's fed from a switched fused spur) - rads all ran fine
Plumber came back out and said a valve was shut on the manifold which he opened - he was concerned about the pressure the boiler was reaching though (around 2.5 bar) so also fitted an expansion vessel above the UFH manifold
UFH ran fine for several days (until last night when I put the heating back on as it was a bit chilly).
With heating on got the E125 fault again.
As for whether the expansion vessel is fitted on the flow / return I'm not 100% sure. I've just got to nip out for an hour but I'll take some photos when I get back and post them up here to show you the set up. Thanks for all your help. Very much appreciated