I actually run a one day and a two day course on heating system wiring. The one day goes through the basic S and Y plan systems, wiring, connections, system control etc and practical workshop time, and the two day adds on fault finding, and the other systems (C plan, W plan etc) and practicals etc. It is not so difficult once you get your head around it, it is in effect a series of switches, which eventually power up a boiler and pump. I notice you are in Brecon, Wales, and as I am in Berkshire its a long way to come!!!
Have a look at Honeywells web-site, lots of free wiring layouts there, and try to follow them through. If you are still not sure reply with your queries, good luck, you will get there!! Not sure on the forums policy about swapping email contacts etc so I will wait to see how you get on. Happy reading!!
