Hotpoint TDC30S belt & motor ok but drum not turning?no

Hi,
Thanks for reading this. I have a Hotpoint TDC30S Condenser dryer that has been overhauled. It was my son’s and the belt had snapped so he fitted a new one but the drum did not turn. I bought him a replacement dryer and thought this would be a quick fix for me and his mum to use as backup to our washer/dryer. The rear bearing had worn and the drum had sagged wearing out the front panel support bearings and their holders!!! So I fitted a new front panel complete with felt seal and support bearings and then new rear bearings. However, on switch on the motor turns as it should but the drum does not turn? The belt is the correct 1701141 and is fitted correctly. There seems to be a lot of play with the two tensioners and if I pull the ‘outside tensioner’ outwards the drum will turn ok. When I release it the drum stops turning again. My thoughts were that the tensioner spring was stretched, but I do not know what it should look like? One of the black tensioner wheels has a well worn groove the width of the belt almost about 1/8″ deep. I would really appreciate any help with this mysterious and seemingly easy component part. Thanks again.
Regards,
Ron.