I think it is entirely fair. First of all, the quotes were not just for changing some "pesky" radiator valves, that was just a small part of the work. I also wanted a new radiator plumbing in to my workshop, a new towel radiator plumbing into a new en suite, I wanted five new radiators fitting, the boiler needs servicing too, I have a dishwasher and washing machine supply and drain that needs plumbing in, and I also wanted a zone valve fitting, along with two wireless thermostats. Not only did I want all that work doing, I also wanted the tradesman to supply all the items needed, (five rads, 13 TRVs, 2 thermostats, zone valve and all fittings). And you think it is okay just to get one, perhaps two quotes for all that?
You said:
I've moved into a house that has radiators with twin entry valves connected to 10mm diameter copper piping. I want to replace these valves with TRV's, so I called a few plumbers in to give me some quotes.
Why are we expected to imagine that you actually meant something completely different? I stand by my earlier post.