Just having a very quick look at it this morning, as it was almost dark when I got it home last night (and I was four beers into my Friday evening). I'm no expert on these, by any stretch, but I think it's a Mk 1. The hub is a four speed and is dated December 1964, and the pedals are pretty worn but I think might be the original items. The brake calipers will be swapped for Weinmann items: original or not, I can't live with those horrible pressed steel things! The rear fork - a weak point on Mk 1 bikes, from what I've read - seems to be ok. There's a bit of play in the headset, but that's easily rebuilt.
I was toying with putting drop bars on this and maybe building wheels to take a cassette and fit it with a derailleur, but the more I look at it the more I think I'll just fettle it a bit and keep it reasonably original. Anyone know where I can get the pedals re-treaded?