Using a Stelrad radiator on its side

If you can find a way to mount it, then it will work. If its a type with heatsink fins on the back / or between panels, then you would probably see a drop in performance since you won't get the same convection airflow though the fins.

A couple of steel rods or strips passed through the existing loops may allow it to be hung on the existing brackets.

Although much less efficiently. Even specifically designed vertical radiators are about 50% less efficient than horizontal ones and this one will be even worse.

Thanks John

What size are you after? Just looked at an Ultraheat leaflet from when I bought mine, and they do a conventional in 700 or 900 high by 200, 300, 400, etc wide.

Just have the water exit at what is now the top and all is well. Use your brain to figure out how to mount it of course.

NT

But if both fittings are in line (i.e. what would have originally been the bottom of the rad) then the water will convect straight up to the return pipe with very little flowing through the rest of the rad body and very little useful heat given off.

There is flow through the rest, just slower than the pump driven flow path. You're right that it's not the ideal layout, but it's workable. End user may need to open the LSV further compared to having it the usual way up.

Yup. Done this in the past when I was very short of cash, still working OK. In a hidden location where appearance doesn't matter. Only you can decide.