Since it will turn if there's no pressure from the other end, I think we can exclude the spindle lock. So follow Paul's advice. Once the belt is loose, the spindle must rotate when even the slightest of force is applied. If it doesn't, you need new bearings. Probably best to contact Axminster for further help in this case.
If the spindle does turn, then see whether you can SAFELY try and turn the other end of the belt drive, i.e. the bit that comes out of the motor. REMOVE YOUR POWER CABLE FIRST! If the motor side gives any noises or more than a mild resistance, your motor is the problem.