Thanks for the advice on posting pictures. With your help, I've figured it out now. Here are some pictures of my trolley.
Before I made it, I did consider the PVC pipe version but I didn't think that would be stiff or strong enough.
With respect to rough tracks etc., I was once on a trip when we had to land (with difficulty) at Winspit on the Jurassic Coast. We then had to carry the kayaks about a mile uphill on a rough track. Some of the guys in the group had Ekla type kayak trolleys, which really struggled. My wooden trolley coped easily. I've also draged it up shingle beaches and used it to carry two sea kayaks, one strapped on top of the other, all with no problems.
I think the key factors for strength and ease of pulling are:-
1. The section thicknesses of the wooden members and axle blocks are big enough to support the load.
2. The wooden blocks are bolted toghether, using 4 (2 per side) M10 galvanised coach bolts.
3. I used 2mm wall thickness aluminium tube for the axle (the Ekla trolleys look much thinner).
4. I used 10" diameter pneumatic wheels. Most kayak trolleys seem to have smaller wheels. Bigger wheels make the trolley a lot easier to pull over soft or rough ground.
As you can see from the first picture, it breaks down into parts, all of which easily fit into the rear compartment of my Capella. I did think of fitting wing nuts to the coach bolts, so I could easily break it down still further, in case I wanted to pack the bits into nooks and crannies to save more space. However, I haven't needed to do that yet.
It could be modified for use with an open canoe, just by adding one more wooden block each side and using longer bolts, to make sure the wheels don't rub on the underside of the hull. I haven't done this yet, as I have an Eckla open canoe trolley. However, being a bit lower down, I think the wooden trolley would propbably be a lot more stable than the Eckla.
I built the whole thing for aboout £10 to £15, although I did already have the wood and varnish in the garage and I used the wheels from my Eckla open canoe trolley. The aluminium tube for the axle came from eBay. The PVC tube for the axle spacers and the coach bolts came from B&Q.
It was pretty easy to make, saved me a lot of money and so far, has really done the job. Many thanks to Jules for the original idea.