Thanks guys,
created a bit of interest with this problem.  I have the manual and bought the 12/6 prop after reading a test report on various prop sizes, but my problem is that if I fit a 12/6 the blade will be grass cutting at  zero ground clearance.
The RCV58 CD has a gear driven shaft and is 1:1 and will take conventional size props. It has no poppet valves or springs.

I think a 10/6 will still not give me enough ground clearance and so I thought a smaller diameter 3 bladed would do the job.
At the moment I haven't seen any small diameter 3 bladed props for sale but I spotted "sports props".. 2 bladed 7 inch diameter with a big 7 or 8 pitch. This type of blade has got me thinking again.. I wonder, could I use one of these ??