Joe, Went to my nearest Mower Superstore today. In a nutshell, because of the wear on my chain they refused to shorten it for safety reasons. They showed me the Stihl replacement and, especially in my ignorance I had no option but to accept. So I bought the new one but argued a discount, - came out at $38. I attach a photo. My woes are not over though - as I spin the chain I have a tight spot. So I have the chain as slack as I dare - the chain hangs about 1/2 inch so that the tips of teeth (that engage in the drive sprocket), are just still in the bar groove - and then when I hit the tight spot i can just about pull the chain by hsnd. I can only think this can be worn drive sprocket. I attach photo.
If you have any suggestions I shall be very grateful. I would like to think that the chain as it is won't fly off and the tight spot will stretch/wear - or am I fantasising.