I think its advisable to have some Professional Indemnity Insurance, to protect yourself against legal action made by your clients. Litigation is becoming more and more common, and although it's normally the accountant the client goes after, I think as a book-keeper it's better to be safe than sorry.
I think the average annual cost for a book-keeper doing basic book-keeping should be around £150.00. I pay about £12 per month by DD.
I use Hiscox. However, there are many different insurers in the market. Google 'Professional Indemnity Insurance+Book-keepers). You should get a few hits.
Yes, sorry, the lawyer in me again. I think you ned to have some type of insurance. If you get a really high deductible it is not expensive and then it becomes more of an emergency policy. Especially starting out, i think its advisable.
I don't have any indemnity insurance either, but I should probably think about getting some because I know writers do get sued sometimes. There are so many things to think about when running your own business, aren't there???