Hello Kevin,
You are looking for 'belt stick', it is a black resin based material that is applied to the inside of the running belt where friction melts it and impregnates the belt. You get a satisfying sticky tearing noise as the belt runs off the slack side of the pulley. It comes in a paper wrapped stick about a foot long.
Try agricultural engineers for it, it used to be available in Yorkshire up to 20 years ago. Old traction engine owners are likely to know of it as well, the ones that demonstrate belt driven machinery at fairs.
http://www.lathes.co.uk is another possibility, they sell flat belt jointing.
Belt dressing is lanolin based and is used to soften old belting, it won't help at all in this case.
Regards
Brian
Edited By Brian Wood on 24/12/2014 10:01:48