One thing you might check is to verify that you have the spring in the carburetor linkage between the carb and the governor. This is a fairly weak spring that connects to the linkage rod about one inch forward of the joint between the rod and the carburetor throttle lever. The other end of the spring is connected to a fixed point up behind the intake/exhaust manifold. It could be broken or missing. I understand the purpose is to prevent surging but I have never tried running my '42 B without it so I don't know for sure.
"whilst ticking over"? Interesting phrase that I haven't heard before. Are you in the UK?