Whirlpool 7244E remove the drum?

The above machine is leaving lint on clothes. I have stripped it down and the plastic drum outer is filthy and looking at the user manual I assume the 'Magic Clean" lint filter is blocked up. Nothing is stuck in the impeller or hoses. My attempted solution will be to take out the metal drum and give it a good clean out. Agitator is out and top & sides are off. There is a large circular looks like a locking ring which I can't budge. I have even made up a tool to try and undo it. The machine is 9 yrs old. Is there some trick to get the drum out or is the locking ring just frozen on requiring a heap of CRC? Please Help.