I have to put in a new cold water tank in the loft this feeds the hot water tank the replacement I have is slightly different to the old one, they are the same size but inlet holes for pipes are in a different place. Can i use push fit fittings and plastic pipe it is copper pipe at the minute and i need to re-route them to fit new tank
Its inevitable you'll have to repipe for a new cylinder. Unless you know what you are doing with pushfit I'd suggest you use copper. Theres going to be nothing worse than mains pouring out into the roofspace coupled with the CWST emptying itself if/when the joints fail. Mind you theres that risk with compression fittings on copper pipe when it's inadequately lagged and freezes.
However if you want you use plastic you could. Make sure you use inserts, make sure the pipe is fully through the O ring to the stop in the fitting, don't use a hacksaw to cut the pipe - use the proper cutter and make sure you clip the pipe as often as possible. Finally DON'T mix speedfit fittings with Hep pipe and vice versa or any other combination.
Is it not worth a couple of hours of a plumber to do it for you? Might just be cheaper in the long run.