It's becoming more and more important to ensure your terms and scope of service are fully understood by both parties (i.e you and your client). The easiest way to do this is to write down EXACTLY what has been agreed. You can easily do this yourself without the need for a solicitor.
If you don't what to include you can purchase a standard book-keeping contract template from The Book-keepers Network shop (www.book-keepers.net) which I use.
Litigation is becoming more common, so I think it really is wise to have something in place.
Just have to say I always have a written agreement in place when I take on a new client. There have been a few instances where disagreements have been resolved by referring to our contract. Worth their wait in gold. Just my view though!