Thanks for your help so far. What I want to do is introduce a new radiator close to the boiler, and two more a bit further down the system.
The boiler is an oil fired Potterton Statesman Utility. It has 4 pipes into the boiler. Looking from the front: bottom right is a 22mm pipe, top right is a 22mm pipe which goes into the pump and comes out at 22mm. Bottom left is a 28mm pipe with a drain tap at the lowest point as it enters the boiler, top left is a 28mm pipe. All 4 pipes come out and disappear into the ceiling void. With the hot water on and the heating off all except the bottom right 22mm pipes are very hot.
Before these 4 pipes disappear into the ceiling void I want to cut 15mm pipes into the CH flow and return for the radiator there. Can I assume that the one through the pump (top right) is the flow, and that the return is diagonally opposite (bottom left) with the drain cock? If I cannot assume this, what is the best way of finding out which is which?
The two radiators further down the system a more of a problem. The pipes come out of the ceiling void about 3 yds away from where they go in. It is pretty much impossible, without ripping out the ceiling, to follow the pipes from the back of the boiler to the other side. There, where I want to cut in the 15mm radiator pipes I get a 'snapshot' of the pipes as they pass. However for some reason there are 3 x 22mm pipes, 2 x 28mm pipes and 2 x 15mm pipes. Underneath the ceiling is a shower, so I assume (views welcome?) that the additional 22mm pipe is bringing hot water to the shower. One of the 15mm may be bringing cold to the shower and the other (I assume) is the cold water coming into the house and going up to the tank in the attic (the main water main comes in in this room). When the hotwater is on and the CH off 2 of the 3 x 22mm pipes are boiling hot, as are both 28mm pipes. I can not immediately see any pipes coming off these. How then do I identify the CH flow and return?
Any clues very welcome!! It might be that I need to employ a professional plumber for 30 mins just to tell me which is what!