Removing/Disabling TRV's

[doitmyself]

I was told that the recommended temp diff is 20C on condensing blrs (to maintain a low return temp of <50C - has this changed?

Not sure, but I know when condensing boilers were first used by BG (over 30 years ago) those older boilers could not modulate so it was a case of fitting over sized radiators and operate the boiler at a much lower temp to increase the time the boiler spent condensing, today its possible to run boilers at high temp as the boilers can modulate.
Martin.

The important temperature is the Return temperature. This has to be less than 55C or the boiler will not condense.

If the flow temperature is 11C higher, i.e. 66C, the radiators will only give off 75% of their stated output. If the flow is 20C higher, i.e. 75C, the output will be 85%.

I can't prove it, but my suspicion is that most condensing boilers rarely condense as they are run at too high a temperature. This is because the installation was not designed with condensing boiler in mind, i.e oversized radiators to allow for the reduced output. This will be particularly true where the condensing boiler is a replacement for a non-condensing.