I'm not really sure what you expected to hear when you posted this, Kevin, but Internet forums aren't really the place to pursue a grievance with a specific trader and I'm sure Colin B would tell you the same thing. Please also don't forget that this Forum is run by a commercial concern and that conflicts of interest may arise.
That said, I gather that a month is not unusual in the case of Speedline Models. The man has been in poor health for some considerable time and his business has fallen behind as a consequence.
First I would suggest asking your bank to confirm that your cheque has been presented. It might not have been received.
You might try contacting Adrian through Model Boat Mayhem, where he posts as 'Flundle'.
The Citizens Advice Bureau can give you proper legal advice on pursuing the matter after that. This generally involves writing a formal letter and sending it by Special Delivery, then waiting until a specified time limit has expired (say 14 days) before taking the matter further.
Dave M