An update to this post...

As I mentioned... I had 'help' with this from a friend and we put a little transmission fluid in the cylinders to lubricate them. It seems it worked, they are operating much better now. Have also spoken again to 'Darrell' at Sierra and he's suggested squirting some of his special grease into the cylinder (he's supplying a syringe and grease with tail wheel retract I've purchased form him) - so I'm very thankful to him for his help. He says that 'most' of the time this is the problem - the cylinders/seals dry out and just need to be re-lubricated. Failing that the cylinders will need to be removed and returned to him for replacement - apparently they are manufactured as a 'press' fit (and probably a special glue) and cannot be taken apart - at least not by a backyard mechanic!

Plenly of time to get them right before they need to go in the model..