Have you tried www.kiddicare.com? Also go through Greasypalm and you get some cashback. You should clean your Marco Sky up an ebay it. You can send it by courier for about £15.
Gotta say, I tried a 3 wheeler and I found that as soon as you get a bit of bumpy terrain - where hedgerows have been cut back and the like - they get really hard to keep upright, I'll stick with a good old-fashioned four-wheeler ta.