Originally I was going to use the wheels off my roofing ladder, but when I put them on the dolly and put my weight on it the M6 coachbolts holding the wheels bent...
Back to the drawing board. I borrowed the wheels of one of those horrid heavy cheapo sack barrows which we have in the garden shed, they are a bit heavy, but should do the job.
I made the axle from some 1/2" ID steel tube with some stubs of 12mm stainless steel bar epoxied into each end, this makes a lightweight but strong axle. I had the materials lying around from my old electric golf trolley which I made yonks ago. Here's a link to a blog I did about it on an engineering website http://cr4.globalspec.com/blogentry/6416/DIY-Electric-Golf-Trolley
I shall strap the dolly to the log with rubber straps and hopefully it should save my shoulders from having lug a wet Yew log 1/4 of a mile.