After lots of wiggling the back wheels backwards and forwards, I finally concluded that the differential is probably a bit slack and could do with replacing.
I have another one that some one gave me about ten years ago and it seems to be a lot tighter so I think it’s worth replacing. However, after spending a day researching about it on the internet, I found out that although they all look more or less the same there are four different varieties. I tried to work out which one was mine by the code numbers cast into it but because I left it buried in a hedge for a few years all of the numbers had long since rusted away. Luckily I eventually found out that you just have to count how many turns of the input flange it takes to turn one of the output flanges and that tells you what type it is. It was the the same type.Now I have a dirty job getting underneath the car, taking off and replacing the old one.
If there was no was internet I would have just done it without all this research and done exactly the same thing.