Dave,
I have a similar trolley to the Peter Fogwill one. Ian_Giles (a mod here) began WFP with one, and he thought it was great.
I personally don't think they're suitable if you're working from a car and still plan to take ladders out with you.
It's difficult to get in or out of the boot, when you've ladders on your vehicle, although you can put it on your rear seat; you won't want to after you've ran the wheels through dog poo. Big and bulky too, but it does carry 50 or 60 litres of water depending on your barell size.
Don't groan, fellow forum members; but if you're just planning to go WFP, on a budget, and still work from a car; go for a Shurflo Back Pack.
I believe there's a number of companies selling 'everything you need' to start you off with a WFP, with the backpack (for around £500 (probably plus VAT and postage too). I think Cleantech and Aquatech (Peter Fogwill) both do this service.