I got myself a flymo turbo compact 380 off my dad - he said it was
probably needing a new belt as it kept throwing the existing one.

Sure enough, every time you start it, it throws the drivebelt.

I purchased a new blade, belt and lid for it to get it all back working
properly.

The new belt was snugger, but it is also throwing when the machine starts.

I've had a look underneath, and with the belt fitted, even turning the
rear 'spindle' by hand, you can see the belt begin to 'creep' off it.

Any ideas how to overcome this problem?

help most appreciated,
g.