Thanks for the tips - I think a framed garage may well be a squeeze for the driveway and there are also logistical problems providing an electricity supply for the 'carcoon' style ones (but how on earth would any of you know that?!). I'm inclined towards the £72 version from morganhoods.com (Halfords offer a c. £75 'advanced' cover, but for reasons I can never quite put my finger on I have an aversion to shopping there).

At least this will give me a starting point and I'll see how it copes with the British 'summer'.