Thanks for all your nice comments guys, much appreciated.
I decided to take a hammer and chisel to the base board of the model and remove enough of it to allow the new flywheel to sit in its bearings.
I was desperate to know for sure that it runs true, and it is absolutely spot on so I am well chuffed.
Here are just a few of what it looks like now, and one with the old flywheel for comparison.
For those that are still wondering, there are 274 parts in the new flywheel.
Phil
P.S.
I have a very nice 11" flywheel that is now surplus to requirements if anyone happens to be looking for one.