An accidental danage fire & theft xs is just that. You will NOT pay an xs if someone claims against yourself and you don't claim on your insurance for damages to your own vehicle. If you claim for either damage to your own car, or if you claim for fire and theft you will pay the xs that's applicable on your policy.
If you repair your car yourself and the other person doesn't claim then if you do or don't inform your insurers (you don't have to) then it wouldn't affect the price of your insurance.
Also...this myth that protected no claims discount stops your prices from rising at the renewal is utterly bollox. When someone applies for a quote the policy is worked out on crime rates, theft rates, accidents in the are, deirability of the car, your claims and your motor convictions. My 'base premium' was �1000. They will then deduct whatever no-claims 'DISCOUNT' you have....in my case 65% and they give me my price of �350.
If you claim and you have protected no claims discount, your 'base' premium will rise but you will still be able to deduct the amount/percentage discount you have to form your new price.
I hope this helps....i'm a car insurance underwriter by the way.