@ Alvin Wyllie
I've done a lot of system conversions over the years and I can't say I've found a particular problem with Danfos thermostats.
I had one house full of rads pop under the higher pressure once (They weren't old looking rads either), rad valve unions and gland nuts have also leaked, but never thermostats (Unless they were already faulty).
Sounds like a mate has ill advised you (Possibly due to their own bad experience) or a heating engineer is trying to put a few more pounds on the job.