UPDATE:
Just a kwik recap - this is not so much a problem with replacing a pump - my Worc-Bosch boiler keeps tripping out on E9 (temperature rising to fast trip out). I was lead to beleive by WorcsBosch it would be the pump, now I'm no expert but I really do not think it is the pump, it works fine between trip outs, and is only 3 years old. Therefore I have been a bit stuck, not wanting to replace part after part hoping to eventually replace the right thing.
Well anyway, I am now able to recreate the circumstances to get this intermittent fault to occur! I am very much hoping it will mean something to someone, and get me a better idea of how to pregress forward from here. OK, this is how to get the fault to happen :-
A) The central heating is ON by itself (I got a 3 port MV btw)
B) Then whilst the CH is still ON, the HW kicks in too, so now CH+HW are both ON.
C) Now whilst CH+HW both on, room temperature is reached, and CH turns OFF, so now just HW is ON.
D) Soon as C happens ((a few seconds later), the boiler takes off, starts getting really noisy, temp starts rising too fast. The fault is reproduced! E9 trip out!
I should point out that the motorised valve is moving from position to position perfectly fine, I can see it moving, and CH and HW work fine independantly, and indeed both together, except when the CH goes off whilst both CH+HW are on at the time.
What can it be? I am really very keen indeed to get this sorted now, and help will be very much appreciated indeed. Thanks in advance.