Halstead Ace High - Overheat and Lockout?

Hi,

We have a Halstead Ace High. Within about 45 seconds after drawing a substantial amount of hot water (two showers) the boiler will start spitting out steam and hot water droplets from the little rectangular hole at the top-left (I've taken the front plate off). It will then usually lock out the next time we draw hot water.

Had a guy in who said it was probably the little switch getting stuck in the 'on' position and causing the boiler to overheat. He was talking about a chrome pin that comes out of the big brass unit near the bottom and activates a relay-type switch inside a little plastic box (sorry, I don't know the proper names). We investigated this and it turned out not to be the case. In fact, the pin does go back (although slowly) and the boiler seems have successfully turned off, but then 45 seconds or so later, the spitting/steaming starts.

We've had an engineer in and he wants to replace the whole Diverter Valve at a cost of almost £300. With a baby on the way we could really do without this!! I know nothing about boilers but am generally pretty smart and handy, so would like to seek alternatives or at the very least understand more before forking out this amount of money.

Would anybody be able to suggest any alternative advice and even possible cheaper courses of action?

Your help is much appreciated!