Found it! You are right, it is an ATCO Groundsman and definitely not a Ransomes 36″ (Mastiff). My memory is a little hazey on the Groundsman- I have a feeling that there were two models, a 28″ and a 32″ cut. Both suffered from failure of the cone clutch next to the engine and were a little lightweight compared to the Ransomes Mastiff.
Thanks for the reassurance – 28 and 34″. Those cone clutches were a bit vicious to use but were ok so long as treated properly but people would over lubricate them and the machined cast cone and concave seemed to absorb oil and then required serious wash in petrol or thinners.