Another plus to Kwik Fit over the other one posted is that the total amount of cover you choose (£300/£500/£1000) lasts for that amount (up to one year); e.g. if you have £1000 cover and your excess is £300, if you have 4 claims in a year the first 3 excesses will be paid and £100 of the 4th will be paid.
Remember that the claim has to go over the excess of your main policy - if you crash and your insurer thinks you've only done damage worth less than the excess, there's no claim under the main policy and you can't then use the excess protection policy to pay out, so choosing massive excesses on your main policy could come back to bite you.