Stu,
Are you handy with a multi-meter?
You need to find the two wires, that when connected together, start the wiper motor on the slow wipe. Probably easiest to identify by testing the contacts on the switch itself.
Then you will need to fit a momentary switch to these wires. This should give a single push-to-wipe function. Pushing the button will start the motor and it will continue to run until the blades return to the 'home' position. Simples .
Edited to add that if Caterham use the standard colour codes, Orange should be the live and Orange/Blue the slow wipe.
Edited by - IanW on 17 Jul 2012 14:42:13