The bike overheats, so i replaced the switch thinking this was the problem, as the fan was not coming on. I have now connected it to a radiator bolt and it now is permanently on, when the ignition is on. Solved the overheating issue, but worried that this will kill the battery. If i push the cable directly onto the radiator, the fan does not come on. Any ideas what could be the problem.

The fan needs earth to run, this is what the fan switch does when up to temp.
If the fan comes on when you earth the wire, the circuit & motor are working correctly.
If the fan doesn't work when you 'earth' the wire to the radiator, then the radiator isn't earthed. They are rubber mounted & should have an earth wire from the rad to the bikes frame/engine/battery or wherever. you may have a loose earth wire dangling that has not been reconnected or corroded off or just have a bad connection

Phil

Phil, thank you. This all makes sense now. I will have a look over the weekend.