So I was trying to replace my rear bearings and got stuck, (see question below):
After this I felt confident to go ahead. Was putting it back together with my son and we had it most of it back together (no cassette on it yet). We put the quick release through the axle and span it, all seemed nice. I held the freewheel hub bit and he span the wheel (slowly) and it made the normal clicking noise as it rotated.
So we tighted the bits and went to put the cassette back on. After tightening the cassette locking nut we noticed that it was not rotating. Tried loosing the cone hubs, nothing, tried taking everything off but still nothing.
I am not sure if I have somehow killed the wheel or there is something I need to do to get it rotating again.
Any and all advice on how to get this working / rotating correctly again would be greatfully received.