Hi - it was my original instructions that inkysloth linked to. If you look at my pictures it's your rubber piston seal that has hardened while in the closed position and will prevent you from pulling the piston out any further that it does at present. You need to unscrew the piston head:
1. Screw your piston knob back onto the pen so it's fully screwed on, and then unscrew the section.
2. there should be the pointy end of the piston that holds the rubber seal, just protruding out of the open end of the barrel.
3. This bit needs be unscrewed from the piston rod while in this position. It's reverse-thread, so you need to hold the piston knob while unscrewing to create the resistance. If it proves difficult to budge, you should be able to cover the end with rubber (I use a spare tube cut off the end of an ink sac) and gently turn with pliers (remember it's reverse-thread!).
Once this end is off the piston rod should be easy to withdraw, as the threaded end is as thin as the rest of the rod.
Re: Pensbury Manor - my personal opinion is that it's not much better than paint, and as with all paint you really need to make sure surfaces and painting conditions are perfect to get any result. Does your pen have a pattern on it? If it's just smooth and the name imprints are deep, you could just polish it with metal polish (eg. Brasso). This removes the brown oxidised layer, but of course also removes a layer of the rubber, so use this approach with caution.
Edited to say I should have read your last post! Yes, you need a new piston rubber, and also a new cork seal in order to make the pen functional again.
Edited by soapytwist, 18 April 2014 - 21:17.