Changing the spring from left to right side of motor is not the answer. Peter ET has the solution which is so simple.
1) take full back panel partially off (pull away from carcus) to allow belt to be put around drum from the front drum seal (you do not have to undo the spindle).
2) Once belt is on the drum make sure the flat side of the belt can be seen and the teeth side is on the drum.
3) Put belt around motor spindle.
4) Pull the motor anticlock wise so the tensioner wheel goes under the belt (you will have to lift the belt over the tensioner wheel).
Therefore clockwise (looking from the back); the belt comes down from the drum under the moter spindle on the right hand side, up and over the tensioner wheel (which is now to the left of the moter spindle) and horizontal to the left hand side of the drum.