I presume he isn't insured? Told you to claim on your own? Shyster!! I'd be taking him to task over that.
Does this one pipe setup have valves that control the CH and HW, a lot of single pipe systems didn't use them and have gravity HW? You mention a gate valve, that would be to balance the flow through the cylinder coil and allow that to be regulated.
If this radiator has always done this then it can only be either a balancing issue or a flow issue. If all the rads and HW are isolated then all the system heat should be channelled to that rad and it should rise in temp quickly, if it doesn't then something is impeding the flow to/into it, with a single pipe system, that is invariably down to the pipework routes/restrictions or a valve issue.
If you take the rad off and then open up each valve, do you get a good flow from both valves? Does it have a TRV or just standard valves?
