We first of all need to look at what an excess is. If you take out public liability insurance, or any other type of insurance cover, you will receive a policy wording and a schedule of insurance.
This can either be in hard copy format or it will be sent to you electronically. Legally, in the UK, you must receive these two documents within 30 days of your insurance cover starting. Or, such confirmation of insurance cover to satisfy you that the insurance protection is in place.
Within your policy wording there will be many terms, conditions, warranties, exclusions and excesses. An excess is basically the amount that you will pay, or contribute towards any claim. Some insurance covers, such as employers liability, do not normally have excesses (unless you go to a real “fringe” insurer, which is usually not recommended).
The purpose of an excess is simply to save the insurers money by not dealing with and/or paying out on small value claims. If you are a carpenter and have public liability insurance, your insurers do not want to pay out because you dropped a hammer and caused £5 worth of damage to a tiled floor.
So, insurers apply different levels of excess to try and prevent you from putting in low value claims. It is very, very important though that you do declare such losses to an insurer. As a worst case scenario, if you as a sole trader have had to pay out of your own pocket for 25 damage claims, all around £50-£100 in the last year, then the insurer has to know because this is worse than careless and only a matter of time before a major claim comes along.
A sensible excess for a trades policy (ie plumbing, building, bricklaying etc) is anything between £100 and £250. Most commercial insurance companies have a standard excess of £100, which can increase to £250 or even £500 if you use heat (ie blowtorches, blow lamps).
Any more than this, then it is time for you to look elsewhere as you really would expect your insurance to kick in for any damage claims over £250.