Steve,
It might be because you're at the computer a bit too much too! Ask me how I know that!!
Get along to the Doc and get him to book you in for Physio - they'll give you treatment, exercises, and hopefully give you a leaflet showing you the right way to sit at the PC, correct height to have the VDU, etc.
In fact, if you let me have your email address, I'll send one off to you that I got a while ago from the Physio I use. I can't PM it cos the system doesn't take attachments, but you can PM me or get my email from my profile.
If you don't already use them, get a hold of Unger ErgoTec handles - they certainly help reduce strain - and try to hold them loosely to develop a smooth, flowing, squeegee action. Don't hold the handle too tightly, make the rubber do the work, not your arm.