In the end, I have gone with International Paints, partly through nervousness about the subject, and partly because the painter recommended it, so seemed like a safe way to proceed - it's "only" the
"above-waterline" that is being done, as this has suffered quite badly over the years, and the painter's team has rubbed down to expose the bare metal in most places... after seeing the preparation time and work they have put in, I am glad I didn't consider it was something I might "have a go at" - so far I am impressed....