Thanks, I'll check those settings. it's got to be something to do with trailer speeds as it goes down to a single '40' symbol when you enter a 40MPH zone.
I've been out for a blast around Berkshire this morning and yes, it seems to be exactly this; I'm only getting the two speed limit symbols appearing whenever I move into a 'national speed limit' area. You then get the faster speed limit (non-towing) appear below the slower speed limit (towing), as per Steve's screenshot in post 1330.
As Tell says in post 1338, you can configure the unit in Settings to calculate routes based on towing or not towing, but I do already have that turned off and it appears not to make a difference. Also, you get the dual speed symbols appear even when the unit is not navigating / following a route. Obviously not a problem in the grand scheme of things, it's just good to know what it is (and, indeed, to have maps that are significantly more recent than the version 0635 which was in the car when I bought it four weeks ago!).