Do you have the stock horn? If so I'd throw it away and get something that makes a lot of noise! I might try running the horn directly from the battery to see if there is something in it that is causing it to draw a lot of amps. Then I would try to follow the horn wire from the plug for the steering column stalk assy to the horn to check for shorts, frayed wires, poorly crimped connectors, etc.
This page (
http://lotus-europa.com/manuals/) has a lot of great information including searchable online manuals, color wiring diagrams, tech data not found in the manuals, useful mods, etc. I think this is your wiring diagram (
http://lotus-europa.com/manuals/misc/electrical/tcfedc.gif).
This isn't likely to be your problem but since you're dealing with your wiring, you should know that US fuses are not rated the same as UK fuses (see attached document). You should make sure you have the proper fuses in your car.
I hope that helps!