The motor is a bit of a puzzle :
Old wiring appears to be three-phase, but the current motor, which looks fairly new, is single-phase.
The motor is numbered 1.541TPCB3, which doesn't appear to correspond with the parts listed at http://www.tecmotors.co.uk/ though TPC is one of their designations.
The exact same motor is listed at http://www.engineersmate.com/products/SINGLE-PHASE-MOTORS-230V/1-5KW-4-POLE-FOOT-MOUNT-230V-1PH-PERM/P239654
The wiring diagram inside the connection cover appears to show a capacitor-start motor with a centrifugal switch, but there is no obvious switch. It would be reasonable to assume some of the internal wiring is to a switch, but the connections shown are open circuit. Furthermore, if they were to a switch, it would turn the motor off once it got to speed.
I'm guessing (maybe over-guessing) that there was previously a 3-phase motor installed and it has recently been replaced by this one, possibly bought from the engineer's mate website as part of surplus custom stock. It is thought to have worked for a while, and it isn't clear why it no longer works. Other documents on the TEC website show a different wiring for similar motor types, suggesting the wiring on this one may be unusual (perhaps it should have an external centrifugal switch?) or wrong.
Possible solutions :
Short, expensive solution - get a heating engineer to fix it.
Less expensive solution - buy a replacement motor from the boiler manufacturer and restore the wiring to original
Other possible solutions
- buy another motor from engineersmate and fit it, hope it lasts longer than the previous one.
- take the existing motor to a motor repairer (there's one near Old St) and ask for an expert opinion on it
- buy a 1.5kW single phase motor with the correct shaft / pulley and fit it. Check that the previous repair has actually provided single phase power (with neutral) and isn't running the motor across two phases