Boiler overheats when only UFH is on

We have a "Baxi Combi 133 HE Plus" and over 5 years ago had a new kitchen built with underfloor heating.
The output from the boiler is T'd with a solenoid valve on each branch of the T. One is the radiator circuit and the other goes to the UFH manifold plus a small radiator. Solenoids controlled by separate thermostat/timers for each zone.

The problem seems to come when only the UFH is on and from the beginning we have had a couple of times each season when the boiler would need the reset button pressed but we could live with it now and again. This season it has been happening every morning - the UFH is set to come on first as it takes longer to warm up. When both zones are calling for heat it all works fine.

When it has happened before I have managed to reduce the frequency of the fault by turning the boiler water temp. down a bit but this time it hasn't worked. The UFH manifold seems to working as does the underfloor circuit pump.

I'd like to fix this issue once and for all and would welcome suggestions as to what can be done to fix it. It seems like the small circuit with just the UFH manifold and the small radiator is just too low a volume for the boiler to cope with but surely if the return temp is not significantly lower than the output it should just stop firing until the return temp drops again - I don't see why it triggers the reset? I also can't see why it should be so much worse this year?

I'd appreciate some suggestions...

Thanks for your time!

No suggestions after a week - guess it's a difficult one!
We've been away for some of the time but having turned the CH water temp down again it seems to have helped as it hasn't overheated in the week since I posted.
I guess I'll just have to continue to live with it.

boiler has a built in bypass and should not require an external one. If internal bypass has become restricted or blocked you could either check & clean internal one or fit an external

As @Daveydub says your boiler has an internal by-pass but this could be restricted and needs cleaning

As above could also be worth fitting a magnaclean and adding some fernox if not already fitted.