Ok, to update, I now have a post up here, and on another mower site, asking for serial numbers of cadets and viceroys (also consul, just to rule it out for certain) and pictures where possible.
I have sent a message asking stationary if he can help, and I have just emailed the power house museum, asking if they have the serial number for their viceroy available!
Hopefully, we can ID the engine, and then we will have a much stronger starting point.