Get the expansion vessel checked.

Water in the rads expands when heated. The expansion vessel is designed to deal with that increase. If the expansion vessel is faulty or needs recharging the expanding water has nowhere to go except out of the "safety pipe" (eg through the pressure relief valve).

A quick check invoves pressing the car tyre type valve on the expansion vessel. If water comes out it's faulty. If air comes out it may need recharging. The proceedure for recharging has been covered in previous threads here.

If the expeansion vessel is faulty it might be cheaper to add a new one elsewhere in the system rather than replace the one you have (which may require a boiler strip down).

If it's not the expansion vessel it could be faulty pressure relief valve.

Right hand side of the boiler, big red thing. You need to follow the manual for re-charging the vessel first.

You shouldn't need a remote vessel as they are quite easy to get at on this boiler.