Simon,
I sent my car to Brian Millers and he did the starter motor change for me. The heat soak in that bonnet must damage it in some way. Incidently he goes in from the side of the engine to get it out.
However a couple of years later the clicking returned, it always started but was preceded by a number of clicks. Batteries were replaced, usually about 1 a year, I have had a few. I always issolated the battery to stop any drain. Ctec chargers did not help save them. I bought a carcoon to keep moisture out of the equation.
I have had my specialist look at connections and electrics and nothing looks wrong. I just live with it and it so far has not let me down. I am convinced the high temperatures must be cooking the batteries but fortunately a new battery always seems to keep the clicking away.