Just to update this, the headlight washers have been re-piped and some previous bodgery undone - there was apparently acres of pipework not doing very much.
Took the car in to Camspec in Crawley today as they fitted a washer pump last year to the car, before I bought it. Unfortunately the pump cracked while they were trying to remove it to test, but because TPS have a no quibble parts warranty below a set value they are covering the cost of a new one
they've also ordered a pair of new washer nozzles as they're not giving a very good spread.
They also spotted that one of my front wheels is lightly buckled
I can't feel it through the wheel so I may just leave it as it is - I'll end up doing it every couple of months otherwise!