Ah, that's where it went! I had it on my watch list for a while. Congratulations! Fantastic frame for 55 quid Looks like the chain holding nipple has been damaged, so you might want to look into it and send it to a frame builder.
The seller claimed full Reynolds 531, if my memory serves?

I'm sure someone will correct me if I'm talking rubbish, but I think before 1978 they used the round badge (which would leave marks on the head tube, no sign of which in the photos). So 78 upwards? But then again, the lugs seem to suggest 1977 C4 and I'd go with that as a guess. Decals could be changed / added later. What is important is that this is definitely an upper tier frame.
Later on (78 / 79) C5 and C4 had: 1. differently shaped and brazed seatstay bridge (more decorative with cutout lugs) 2. Cutout seat tube and head tube lugs 3. Cable guides brazed on the BB shell 4. more sophisticated seat tube lug bolt mounting 5. Differently shaped (concave) seatstay tips.