Yes, they were pricey even for those times. I was surprised when I saw this add that Jack had posted on the forum.
I might be able to obtain a sulky. It is missing the two wheels but I should be able to find aftermarket ones to fit. The problem with Rover is they made their own special wheels which were very low profile compared to modern ones of the same size. Luckily you can still get the tyres and tubes, but the hubs are a problem. I require two if anyone has some to spare? Any wider than about 40mm and the locknut will not catch the thread on the end of the axle! I've thought about buying trolley wheels and taking the bearing out of the offset, then cutting back the offset and re-inserting the bearing.