Thanks Jim, can't tell if mine have that rising profile upto the steerer tube, I'd have the pull the crown race off. Can't do that at the moment.
I'm thinking it probably does but then has a mismatched crown race making it sit slightly higher, thereby open up the gap when installed. I believe these headsets very prone to failing and spares are almost impossible to find.
I have arrived a fix via some 3d printing and recycled lip seal.
3d printing on ebay £6 plus postage, 2 pieces - the band cover for the top nut, very thin, so slightly stretchy, fits nicely
The lower piece being a simple plastic spacer ring 48 od x 42 id and 2.3 mm thick, then centred on the crown race via trimmed down lip seal (likely from a bottom bracket or inside of a free-hub).