Thank you all for your help. I had already tried bendiing the tab on the joiner but didn't want to force the joiner off as it looked like it was fixed to the bottom of the rail somehow. There is a "dimple" near the tab which seems to be there to keep the joiner in the correct place. I had another look this morning and gently pulled the rail with the joiner upwards so that the tab came out of the slot in the sleeper. I prized the edges of the joiner outwards just a bit and it then came off the rail. The only downside I can see is that you can't get the tab back in the hole in the sleeper when you try to put the rail back. I had to bend the tab flat and slide the rail back into the sleeper base. the tab seems to be there to keep the rail in place but as it will have another track section connected to it I can't see that that is necessary. Anyway, on a garden railroad you need some room for expansion and contraction so the fact that some rails will slide a bit should be an advantage.
I have some lengths of track on order from Tenmille and some of their rail joiners, their adapter joiners work with Piko and Bachmann track and I think they will act as insulators if you leave out the metal strip. I will experiment when they arrive.
A bit of background; I have been modelling in 00 and 0 indoors ( well in the shed ) for a few years now and always wanted an outdoor railway. An impulse buy on ebay 5 years ago of a Durango and Silverton 4-6-0 in bumblebee colours set it all off. I have been gathering stock, mostly second hand and also making freelance British narrow gauge coaches and wagons from IP Engineering kits. So the line will be an American Railroad some days and British on others. I have about 20 feet of track laid so far and have started landscaping with rocks to make a gorge for the line to run through and have completed some planting so it is starting to look good.
I can't remember how to put pictures in, I just tried but it asked for the URL?
Many thanks, Chris