no worries, I only found out myself 2 weeks ago, and I did also drop a nut while putting it back together
(they will be 5/16" imperial thread)…grabbed a spare of my wreck
my advise is to carefully stuff fine cloth into the area directly below, this should trap any parts that drop…use tape or magnet…I have a fine-tipped magnet, especially for things like this
dont drop that roll pin, or it will be a trip to the roll-pin shop 
Right, got the pin out. It was very stuck!
I tapped it with a hammer first to get one end flat onto the collar, then held my left index finger against the pin at the back whilst holding a cut off nail with my thumb and middle finger, and with the other hand I tapped against the pin. Then, when the pin at the back felt about half way through, I turned the collar around and pulled the pin out with pliers.
Tomorrow hot water to soften the rubber and some vaseline/WD40 to slip it into the groove and then put the whole thing together again.
The other side is still OK and will have to wait until it starts leaking.
Thanks for all the pointers.
Paul