If they fall and suffer damage while they are on your property they can sue you. They may not have much chance of success but, if they go to a solicitor it is likely they would issue proceedings.
What can you do about this.
1. Ask to see their insurance. They should have insurance, which would mean that in the event of an accident that would be the first port of call for a claim. While most people probably don't do this I think it is a perfectly reasonable request.
2. You could ask for an indemnity on their insurance. This means that if for any reason a claim is made against you, their insurance will protect you. This is probably overkill, and their insurance company may not allow it, but no harm to ask.
3. Your household insurance probably covers you for public liability. It may be a good idea to check this with your insurance company, and to ensure it covers tradespeople coming on to your property.