I bought this belt after my 7 year old Black & Decker GL701 strimmer belt failed. Having read other reviews I checked the state of the drive pullys and saw that the large plastic wheel was badly worn. Sure enough, the new belt failed after about 5 minutes of strimming.
This wheel is part of the motor assembly and not available as a replacement part. A new motor is priced at around £50 so not cost effective to replace the motor as a new 550w Black & Decker strimmer retails at less than £50 anyway. Reccommend that you don't buy a rplacement belt if the drive wheel is badly worn (see my picture on the product page).