Thanks for that. Anyway, back to the original question .... surely
someone in the wide world that is uk.d-i-y must have a "Wickes Twin
Impeller, Product No. 414-805. 2A, 300W Max". ... pleaase???

Yes I have - and no there's nothing that can be done to slow it down by fiddling with the
internal wires - only thing I could suggest you try is a cheap variable transformer -
can't remember what they are called - variometer or variac or something - and reduce the
supplied volts a bit, to say 160 to 200 v a.c. , trial and error.
Depending on how the motor is wound something as simple as a diode in series would give a
series of chopped sine wave pulses, but I think that might damage the pump.
You could always just put a tap in the out put lines or an isolating valve in each leg and
slowly wind them towards shut until the pressure has reduced to stop the leaking..
Nick