The Multisync monitors I was referring to are the old ones that can do everything from CGA up to VGA. They are sought after by users on the likes of The Vintage Compute forums. A quick google brings up the Multisync 3D as a possibility, compatible with MDA/CGA/PGC/EGA TTL RGB as well as analogue VGA.
There's a table
here which lists scan rates supported by a number of monitors. That might help. At the moment I don't have a BBC (one is on my wish list), so I'm more interested in something that will work with my Apple IIe RAMworks RGB card and my Sord M23!