For the "other cars" extension to be valid, I believe the car must also be insured by the owner.

If you are driving a car under the clause it isn't insured by your policy - the only cover provided is for third parties under Road Traffic Act cover.

So if the bump is your fault you have to sort out the damage to your mate's car at your own expense.

If the bump is someone else's fault the owner is insured under his own policy and he can claim off the other party PROVIDED you were insured to drive the car legally. So if you aren't a named driver, you need to have the "other cars" clause on your insurance.

If the car didn't need its own insurance, we could all register our cars to our mum and have really cheap insurance on a banger and be covered to drive the nice ones.