update
stripped the bike last week and replaced all the parts
first I took the cassette off and found the free hub to be badly worn and damaged on the splines ,I'm pretty sure the internals were damaged too as the chain had started slipping randomly, had to go to the LBS for a replacement with some new bearings and cones
rebuilt the hub and fitted the new xt cassette , then I swapped out the bottom bracket & chainset
next I swapped over the derailleurs which was pretty easy apart from the instructions being a bit confusing, I then put the new chain on using the largest sprockets method to get the length
after that I put the new levers on & replaced all the gear cable casings and fitted the cables, again quite easy
that was it all assembled and I only had the gears to set up now, except I had a small problem - when I had pushed the connector pin into the chain I didn't notice it had gone in slighly wonky and bent the link
I had to remove a link or 2 which made the chain too short but I thought it would be just to get the gears setup
cue 2 hours of me sweating my nuts off trying to get it working smoothly with no luck, it didn't help I was having to do it all with the bike upside down (I knew I should have got a stand)
In the end I got fed up so took it to the LBS and bought a new chain then left it with them to sort it all out
anyways its now up and running and its very very smooth, such a vast improvement even from when the bike was new
the gear changing is so quick, smooth and quiet its a joy to pedal
many thanks to everyone that helped!