Hi, further to my post about a trade in and mechanics, here's the full spec of my steed with pics.
The cassette wasn't fully secured to the freehub which caused damage to the grooves. Then when the mechanic tried to replace the freehub he couldn't remove it from the hub, which means the hub is probably finished.
There's also a shifting problem where it won't go from middle chainring to large - I have to go to from large to small and then up to middle. The mechanic suggested it was something to do with the position of the rear mech hanger but who knows.