I think it's an F1, though it's hard to tell from the photo. The F2s have a slightly dumpy look to my eyes. The drivers are certainly way too small. When numbering, be careful, as there were several versions of the bunker, including an extended version. The coal rails may also need attention - I strongly suggest checking with a photo of a particular prototype if you can. Good luck with it!

Cheers for the advice- quite a bit I was ignorant of there.  I think I'll need the appropriate Green Bible or Yeadons for it (since I started using the RCTS series I've found them absolutely invaluable). 

Two ways to deal with it- a 'quickie' job involving new wheels, new chimney and a repaint, to create a 'general' model of the class, or a more involved job finding which of them were sent down to Marylebone ( I know somewhere I have a photo of an F1 on a stopping train somewhere on one of the GC-joint lines) and creating a model of a specific example.  I think I'd be happier with the latter option- but it will be sometime late this year/ early next year before I start it at any rate- plenty of time for research and thinking.   

The green bible turned up today and it looks like I have a reasonable choice of three identities I could go for- a bit of modellers' licence to get them down to Marylebone instead of working push-pull trains on the Met/GC joint and we're good to go.  I've ordered some M7 wheels- identical diameters for both leading/ trailing axles and drivers- and decided to swap the wheels over, add push-pull gear and a repaint... and leave it at that.  The flowerpot chimney came in from 1925 onward so suits my period- and why change it if it fits in?