Hi everyone,
Well, I went to the recycling centre on the weekend and picked up a bike chain tensioner. After much mucking around with brackets and bits and bobs, I finally found a simple way to mount it that works extremely well. I did try using the double cogs that came with it, but they obviously didn't fit the chain and would bind up, so I whipped the wheel of my standard tensioner and used that. It makes the chain run smooth and quiet compared to before.
There is still a little roughness at the engine drive gear, but this is probably the rather square toothed cog. Maybe I need to do a little filing.