Dual fuel radiator

[bengasman]

... I would just like to have an independent view on exactly what has happened and also know the correct procedure for future use.

You have come to the right place. Indepencence guaranteed as we will not benefit one way or the other, no matter what has happened, or what you decide to do about it.
Correct procedure is simple. Replace both valves; one should be a manual or a trv, and the other a lockshield. The lockshield must be set and made unaccessible and the other can opened and closed at will

. For example, does it matter which valve is turned off before switching on the element? And how would I know the difference?

Yes, it can make an enormous difference. The lockshield is used to balance the radiator and is often quite critical, and should therefore not be touched.