[Closed] Xt rear mech- help pls!

Checking the bike tonight before a ride tomorrow. Noticed that the silver lower arm of my xt mech isn't really springing back properly meaning the chain isn't tight..
Was out for a quick spin earlier in the week and noticed the chain was a bit loose, thought it was just cos it was wet and frozen..
Had a good look at it tonight and it's really hard to move it manually, never mind under shifting..
I had prev used a bit too much finish line on the chain and had to remove it from the cassette with a wire brush.. Wondering of this might be the cause of it being so stiff/sticky? If so, how the hell do I clean it out so it moves freely? Went to town soaking it with GT85 and took the inner plate off for access but hasn't seemed to work.. I have muc off, three in one oil and petrol- what do you reckon might be best?
Ta in advance!
Al.