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?