Ok,, Tony has taken it apart and cleaned it up and there is a 'flang' that is an integral part of the upper arm and a corresponding one on the saw which isn't shown on any parts diagram I can find This 'flange' is unevenly worn on both sides. When a little sideways pressure is put on the upper arm the saw works perfectly so it seems that it has worn slightly and unevenly. Tony, being an engineer, says that it is a design fault. The tensioning spring is on the side of the upper arm and over the years that slight 'pull' on the outside of the arm has created the uneven wear on the flanges.
He's going to see if he can flatten the surfaces of both flange to make them run freely together. However, it would appear that my saw is way older than I was told it was when I bought it. It was second hand.
The 'upside' of all this, I will have to buy a new saw !!! So now I just have to decide which one