why does my combi boiler run a full pelt?

I have an alpha 240 eco combi boiler. On Monday i came home to find the heating had stopped working. On inspection the light was on indicating there was a problem with the flu or fan. On inspection there was no problem with either; however i waggled a wire or two and the light turned off.
Then when i put the boiler back together i turned the boiler onto water setting ready to light the pilot light. As soon as i turn the switch to the either the water or central heating setting the pin pops up out of the diverter valve and pushes the primary flow micro switch. This then causes the fan to run at full speed. Once the pilot light is a light and i let go the gas releases and fires up to full. This happens even when there is no hot water tap running or the heating is off. The burner does not stop it just keeps running at full pelt.
I don’t think the problem is the DHW flow micro switch as I have tried disconnecting it and the same still happens. Has anyone any ideas what could be causing this, any help would be greatly appreciated.