grant combi boiler not firing

[dw50590]

Hello there, this is my first time on this forum so pleasego easy on me. I have a grant 90 v3 oil feed combi boiler and have been havingsome problems with it for a while. The problem I am having is the boiler is notfiring up when I use my hot water however if I switch the boiler off and backon again it will fire up fine and will stop when it has reached the required temperaturebut as soon as I use the hot water it fails to fire up. I have read somearticles on this subject and people have put it down to the diverter valve andthe micro switch not engaging. I was just wondering if anyone else can shine anymore light on this subject.
Thank you Adam