I have all those items so will give it a go once the sax is stripped. Hmmm... before or after it's been to the polishers?
As for Plus Gas, Google leads me to a product called "Shock & Unlock" at Halfords:
Still can't get the MP off. Tried freezing the crook... no good. Tried WD40... no good.
I would say after - you're going for a bare brass finish so a few fingermarks aren't going to matter, and you won't run the risk of a polisher undoing all your hard work.
Shock & Unlock is great stuff - but if you're not tackling the screws immediately then you're wasting half of its potential (the freezing action).
It will take time for the WD40 to work on the cork...but you can also try hot olive/vegetable oil.