I finally got around to checking the filter 'sock' in the tank. But there was no 'sock' in there! All there was, was a small clip-on cylindrical plastic and gauge filter.
I took it off and blew it through with carb cleaner. It wasn't very dirty anyway, and the rest of the tank seemed fairly clean, so job done. Well, apart from one of the captive nuts on the rear seat bolts that just turned. And of course, it was one above the tank. Fortunately I managed to lift the end of the seat up enough with the bracket bending, to access the tank.
I took it off and blew it through with carb cleaner. It wasn't very dirty anyway, and the rest of the tank seemed fairly clean, so job done. Well, apart from one of the captive nuts on the rear seat bolts that just turned. And of course, it was one above the tank. Fortunately I managed to lift the end of the seat up enough with the bracket bending, to access the tank.