So you need to split a 15mm central heating pipe?I'd be using on of these.http://www.screwfix.com/p/equal-tee-15mm-pack-of-2/95632Thats a crazy price though. You need an insert to put in the end of the plastic pipe for the olive to grip on. Then just get a 15 to whatever you need reducer, to go in the cross pieces of the T. At least it is easily serviceable.Post overlapped.
I've just checked the pipe that I have and it's 11mm ID, same as Gem's.