Just an update on how this project is going.
Received my new carby and fitted it to my mower. Had a bit of a hiccup in that the OEM fuel tap didn't want to screw into the fuel inlet port on the new carby. It was a coarser thread than the original and after visiting a couple of places to try and find a fitting a bloke at Enzed found a 1/4" BSP fitting that while not the same thread exactly still wound far enough in to make a seal.
After fitting the carby while I was messing around the throttle cable decided to snap on me. Got a new one from the mower shop and paid more than I should but didn't care, I just wanted to get it done.
Set the high and low metering screws one and a half turns out and the mower fired up straight away. Tuned both screws until it was running pretty well and then went out for a test run on the front lawn.
It's running really well now, no breaking down at high revs under load and I'm really happy with the way it cuts.
Thanks again for everyone's input in this thread.