Thanks for the suggestions and advice, everyone.
I should have mentioned the figures are glued 8 to a base, and are gloss painted (I know, not popular nowadays, but I like the look when used with the formal poses of 25mm Minifigs). I'm not confident that I could successfully shield the figures from any kind of spray sealant. But certainly I could use spray when it comes to scenery and terrain.
I looked up "matt sealant" which led me to 1 litre containers really meant for sealing floor tiles and similar things. But there was also a couple of smaller craft items – eg "Mod Podge" which seems to act as both a glue and sealant for decoupage.
I've got PVA glue which I've used to stick the figures to the bases. Perhaps it's the particular brand I've got but
when dried there seems to be a slight shine – satin rather than gloss. I've not tried it diluted with water, so this may make it more matt.
As I live near a DIY store, I'll have a look for the Ronseal Mattcoat Wood Varnish.
Anyway, thanks again everyone. I'll maybe try flocking a few bases without putting any figures on them and try the suggestions above to see which works best for me.