Hiya Simon, this is a bike chat forum so you may not get much of a response here. However as I do drive a car & maintain it, I would check the battery 1st, make sure it's pumping out 12volts & it has good connections at the terminals.
If the battery checks out then look at the solenoid, as this like a switch between the battery & the starter, you can bridge the two connections with a screwdriver to see if it's banjoed.
Last thing is the starter but if other two checks you did were ok then this is probably the faulty part.
steo, June 2009
