Update.
Fitted the new idle control valve, started the car and the tickover settled immediately to just under 1000 rpm. Previously it would shoot up to 1800 rpm and then cycle.
Went for a drive and all seemed good until I came to a standstill in traffic, the revs don't go above 1000, which is a big improvement, but after a few seconds they do start to cycle and eventually the car stalls.
I've been advised that it may take some time for the system to stabilise so I'll just keep on driving it and hope is sorts itself.
On another subject, I'm pleasantly surprised how positive the gearbox is and overall I'm enjoying the car.
