Well, following all your very helpful advice, I got the'scope and it turned out that, in fact, there was a flaw in the focusing mechanism. I sent it to Paul Burke in Birmingham (see related posts): what a lovely chap he is! He fixed the focus, serviced it, replacing seals etc, purged and re-gassed it for £45.00 plus p&p (which the vendor has kindly deducted from the price). Incredible service!
Brithunter, thank you for your advice about "moving image" etc. It went on pretty "true" with only a little adjustment required. I now have a beautiful sight and anyone who thinks 40-year-old German optics are a waste of time ought to come and have a peep through this one ...