OK, easy one first.
You have most of the info on the later frame: LL951049 is a TRI800 originally made for a chap called Gareth Hammil, and it is made of Reynolds 753.
The earlier frame is a lot more difficult because any frame made before 1988 is not in the book. I have two of Dave's early frames, one from around 1975 and the other a little later, both are completely different in their style of construction and yours is a different style again. I'm afraid I don't know who made Dave's frames for him back then, I was told Argos Racing but when I spoke to them they said they made some of Dave's own racing frames but not those he built up and sold.
One of the other guys may be able to shed some light, but I think there were quite a few frame builders back then who would supply local shops with frames and forks, these would then built up and sold by the shop under their own name. Regardless, if it is a Reynolds 531 frame it will ride really nicely, one on my early Lloyd's is 531 and I also have a Keith Coppell in the same tubeset and they both ride well. Enjoy.