Thanks Jamie... I don't have an existing belt or it would be easy and "There" as the Prince of Denmark noted "is the rub". Mind you he was more concerned with slings and arrows than with taking an alternator off and realising too late that he needed a new, shorter, belt. Ah well off to irritate the chap in Camberly Autofactors tomorrow, again!
If you have any trouble getting that part number, go to your local branch of Bearing Traders Ltd and tell them the length you need they will source a suitable belt. As I use a shorter belt than the standard, I had to get one from them and found them very helpful.