Simon,
Go for the Penrose XXL barrow platform. I have had one about a month now and it's a great piece of kit, both as a barrow and as a platform. The standard 26inch legs are great for most of the river venues I fish, although I will get a longer set for when I want to wade on the Kennet. It is well engineered and should last years. Plus you can get all manner of add-ons if you so wish.
The only "problem" I have is that my Brilo box is not big enough to fill it up, unlike a Rive box! Oh and you need a car with a BIG boot. I hear that the future XXXL version comes complete with tow hitch and tail lights!
Stu