I went on the Vokera course today funny enough which was very good. VC is a very nice and knowledgeable fella. I (breakdown novice) learnt quite a bit from today.
The sandwiches were very nice too :D
If fitting a TRV on the return then you need a bi-directional TRV, the body will have arrow's pointing in either direction.
I always try to fit TRV's on the flow.
The ones I suggested shouldn't require draining down if your rad valve holds up. But any money over time the isolating tails won't budge and will stay locked open + will probably leak.
Never used those before. I know you can get isolating radiator tails. I Think Wickes do them. Don't quote me on quality though. I'll see if I can find a photo or a link.
This is what I'm going on about - www.lift-offradvalves.co.uk
Could be a quarter turn valve so it only turns one way. Ballofix are like this. Sometimes the shank that you turn to open/isolate will protude further than others and will generally be more a rectangular shape.