Valliant engineer has just been and it looks like my £300 D&G insurance gamble has paid off. He replaced the pump, replaced the stuck diverter valve and it didn't make any difference, no flow from boiler and temp going up to 111 degrees, steam coming out of pump bleed valve like Thomas the Tank Engine "well I've never ever seen that happen before" he uttered in a tolerable Droitwitch accent. He's now ordered a new main heat exchanger and a also a plate heat exchanger and will be back Tuesday to fit.
Only thing he requested is that the black 16 yr old water was drained out and re-filled before fitting the new bits. So my suspicions of it being H/Exchanger have been confirmed and it looks like our trust old boiler will be virtually as good as new when he's finished and should live for another 16+ years :)
Winner winner vegetarian dinner.