Hi Lynne,
If it's for an ongoing period it's unlikely you'll get a fixed fee, probably an hourly fee would be more realistic. Likely costs are £10-£15 per hour I would think. It might be better for you to have a think about why you need one because it may be possible to simplify things by using software etc. There are a few very good web based accounting system that may fit what you need. It could then just be a case of getting someone to look over things every few months (maybe every vat quarter).
What systems do you have in place just now and what aspects are you finding it hard to deal with?