Just had a call from a chap called Roger Fellows. He has a SQ4 for sale. It was his fathers and he now has it. He thinks its a 58, although he would need to check the V5 to be sure. Its not been ridden for 5 or 6 years and currently is a non-runner, although he thinks he could get it going with an hour or so work.
Bike is in Kendal (Cumbria) at the moment. (Although he can mentioned taking it to Nottingham this weekend if necessary).
Contact numbers are 07740 465653 or 01539 733251.
Roger didn't give a price so I suggest if you're interested give him a call for a chat and see how you get on.