Wardy: welding plastic is pretty easy (search for plastic welding on youtube). All you need is some spare plastic of the right type and a cheap (large tipped) soldering iron which can be had for a tenner. Just make sure you fill the tank with water like the chap does in the video so you don't accidently blow yourself up.
If you've got a split tank no harm in giving it a go, even if it doesn't work for some reason you haven't lost anything..
