Is your site very hilly?
I hay on the farm and I pull a small square baler behind a 4010 and a thrower rack behind the baler. This means I have three pivot points - tractor to baler to rack pin to rack steering mechanism. My farm is VERY hilly. I generally NEVER turn downhill on slopes if there's any bales in the rack. If I'm not careful and turn downhill with bales in the rack, the weight of the rack will push the baler sideways. The tractor weighs over 10,000 pounds and has plenty of weight to hold back a 5000 pound baler and a 2000 pound empty rack, but then I start tossing 50 pound bales into a rack that will hold 180 of them. At 60 bales, my rack weighs as much as the baler (and my rack, bales, and baler weigh as much as the tractor). Even at that weight, the rack has a huge advantage over the baler on a downhill turn. If you think about a downhill turn, imagine the point where the tractor has completed the turn and the baler has just about completed the turn, but the rack is still in the middle of the turn. You have a unit that is pointed downhill that outweighs the baler AND the baler has a pivot point behind the big weight of the tractor. Yep, the rack will push the baler sideways until it jackknifes into the tractor's tire. I have hillsides where I won't fill the rack, it's just too dangerous. The rack and baler would outweigh me by far too much to make any kind of turn. My F-I-L once turned downhill with a rack of hay and he ended up rolling the baler. He's the only guy I know that can roll a small square baler!
A second issue with adding pivot points is that it makes it much more difficult to back up. You now need to aim the rear unit and still keep the front unit in line as well. Some people are good at backing trailers, far fewer are good at backing wagons. This unit would make your trailer into a wagon.
Now, with you standing on this unit, you and the cart are going to weigh a little better than 250 pounds, depending on how much you weigh ( I'm not gonna ask...). The tractor weighs 275- 375 pounds, depending on
ballast. So, you can haul 500 - 600 pounds, right? Sure, on nearly flat ground. But, on a hill on this cart, you've added a pivot point between you and the tractor. With 250 pounds on the trailer, you're going to be the baler in my example if you turn downhill. A trailer that is hitched directly to the tractor with you riding on the tongue or front of the box is generally going to be safer to operate.
If your lot is generally flat, you're good at backing wagons, and you aren't going to load the trailer very much, this would probably make a very good attachment. Unfortunately, I do not know of a dealer selling it. You could call or email Tracmaster in the UK and see how much it'd cost to have it sent over. You could also buy the transport sulky and put a hitch on the rear of that, but I'd call that more dangerous because you're sitting. At least with the platform, you're standing and can leap off and out of the way if a situation becomes dangerous.