Hi OBK,
Yes Rainbow will do you insurance for a one off payment rather than hourly rate, just see Jenny or Matt on the front desk when you fill out your hire form - they will probably remind you about insurance requirements anyway. Its a pretty standard thing and covers you for any damage to the aircraft except negligance - presumablly if you start pulling aeros in the A3 that sort of thing
If you email them they are very comprehensive at answering your questions, normally respond to by next day - email usually sitting in my inbox when I get into work in the morning.
Julian.