No, the radiator didn't heat up at all... the pipe to the TRV did heat up when I opened the bleed valve but no heat got into the radiator.
As far as I'm aware I haven't capped off the return from the existing radiator. There was a T at the flow end... One branch going to the TRV of the radiator I have kept the other going to the one I removed and another T at the flow end one branch going to the lockshield of the rad I have kept the other going to the one I removed. I replaced both these Ts with elbows, cutting off the branches to the radiator I removed. Does that sound right?
Thanks again for all the advice - for those that are interested and in case anybody has a similar situation ianmcd was right. I got a plumber around in the end as I wasn't too confident with unblocking the pipe and he was pretty stumped by this too. As I mentioned there was a T at the lockshield end - one branch went to the radiator I removed and the other went off into a cavity wall (I was actually pretty surprised to find this T when I first lifted the floorboards as I just expected there to be an elbow going to the radiator I removed). We couldn't work out where the pipe went after the cavity wall ... we thought either the room next door (a tiled kitchen) or straight upstairs + back to the boiler. I then asked him whether it could be a redundant pipe so we capped it and I connected the lockshield to another return. It now all seems to work so the plumber reckons the pipe must've connected to an old external drain cock. We couldn't see anything outside as a conservatory has been built over this!
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