I replaced the belt - but it won't tumble with a full load of clothes. I'm thinking that it's missing a part? there is a wheel that the belt goes around and then the belt goes around the plain metal shaft of the motor. We have no schematics to know if we are missing a part? It will tumble with one or two items in it
My guess is you haven't got the belt onto the tensioner correctly. Should make an S shape around the combination of the motor spindle and the tensioner pulley. If it was dead easy to fit the belt to themotor / tensioner you did it wrong! See my answer to "belt change on Asko 7305" for more.