Hi darbo, I know exactly what a diverter valve does and I'm not questioning your competence. All I'm trying to get across is that Ideal Boilers, particularly the Isar, Icos and Europa range are notorious in the trade for being b*gg*rs to repair, there may be two or three different variants of diverter valve, hence the need for a serial number. In the course of my work I meet literally hundreds of plumbers and heating engineers and many of them wouldn't touch a faulty Ideal boiler with a long stick.
gucciman is quite right, you could end up worse off than when you started, so it would pay to get a pro in to do the job and leave the headache to someone else.