Thanks Richard, I hadn't come across ACV in my searching, though they appear to be more expensive than the other two brands I mentioned previously. I can understand why that'd be the case when compared to the Elterm Mars, which isn't electronically power adjustable or triac controlled, but it seems a pretty similar product to the Elnur CM15.
As for the tariff, it's a bit of an unusual situation, in that the electricity is free, for the next few years at least, as the costs are included as part of the lease. It would be a bit short sighted to choose electric heating for that reason alone, as power isn't always going to be free, but it's the only source of fuel available. A heat pump would be preferable, but it would have to be air source (it's an upstairs commercial building with no outdoor land), and budget is tight, given that we're talking about an old art-deco cinema that had sat dormant for twenty years, prior to being taken on as a volunteer driven restoration project. That's also the reason for choosing a wet system over direct electric heating, as the original cast iron radiators are going to be refurbished and used again to heat the lobby and bar area.