Ok, i've just got back in from my garage after tapping and weighing my bike. Firstly, I tapped the top tube and must confess, it didn't give the ring of a thin walled tube! However, when I flicked the vertical tube (one where seat pushes into) it most definitely did have a nice ring to it! Not sure what that means...maybe someone can offer some advice?
I also took the opportunity to weigh the bike using a set of bathroom weighscales. I first weighed myself, then me holding the bike. I did this a number of times, as the readings were not exactly the same after each weighing. After averaging the weight readings I came up with a figure of 24.5 lbs. Is this good, bad or indifferent? I haven't a clue!
In answer to the other questions, there are no obvious markings of any sort that I can find on the frame. I have owned this bike since the mid to late eighties. I bought it off a work colleague who, due to ill health, had no reason to keep hold of it. I don't know how long he had owned it, but he used to be a keen cyclist in his youth. Unfortunately, he went to the 'great velodrome in the sky' only a few years later, so I really dont have any history about this bike.
The only date I can find is on the rear Weinmann brake caliper. This is dated as 1979, but I really think the frame could be much older.