Hey chaps. Thanks for all your excellent advice. It is much appreciated by me and has saved me a lot of unnecessary effort. I have just succeeded in releasing the stuck SA gears.

Firstly I used a mini funnel to pour in some uncut ATF and left it a couple of days.
It was still stuck. So I thinned the ATF with mineral spirits (UK White spirit) and poured in a lot more. I spun the wheel a few times and left it a couple more days.

Today it was still stuck. So I squirted in a generous amount of aerosol PlusGas using its small bore tube attachment. A few minutes later, when I tried to turn the pedals to spin the rear wheel, I noticed that there was no drive. So I started to work the indicator rod and it moved in and out becoming quite free. I reattached the gear change cable and it all works fine. Just like new.

So, by being patient and letting the lubricant penetrate, I did not have to resort to using a hammer or to dismantling the gearing which I had been prepared to do. I must credit the ATF because I do not think that the PlusGas could have had such a dramatic instant effect.

So thanks again chaps. A perfect result!