Hi. Thanks for the informative replies; I had so many, I didn't know
which one to quote.

I have ordered an IR thermometer courtesy of a well-know auction site
as this will be the quickest and easiest way to measure the
temperatures and check the system is balanced. Like someone said, a
little money spent on the thermometer now might save money on upgrades
later.

I made a mistake by mixing my BTUs and kWs. The larger radiators at
Screwfix have outputs of 6000ish BTUs, which is 2kW-ish; I was
thinking they were 6kW radiators. My mistake, sorry. So I have more
capacity in my pipe than I thought, which is good news.

The radiators in the house are old, single, without fins, and painted
over. Heat outputs unknown.

It's getting a bit late, so I'll reply to Fred's questions in the
morning. Good night.