Hello Pete
I am far from impressed with earlier Yamahas - especially mid 70's refurbs and the way they are being promoted as professional instruments at half the price of a new one. Their design could not be further from a new Yamaha U1. The Petrof is nice - I saw the new range at Frankfurt Musikmesse this year (April) and was impressed by the tone... a Renner action is a good idea as my only gripe with modern Petrofs is the Detoa action than has been highly problematic over many years. Having a Renner action in there overcomes this concern.
Although the S1 is 118cm, and therefore 3cm shorter than the U1, the early U1's had a poor bass register, so the inefficient design of the early U1 contradicts the height theory.
A modern Yamaha - post 1999 vintage, is a different matter entirely...
Models B3, P121, U1, YUS1 all have the current spec. U1 design benefits over the earlier models. Curved bass bridge (increases length of mid and upper bass strings), full perimeter frame (more mass) wider frame (longer bass strings and increased soundboard area), 5 backposts and 2 tone collectors (greater rigidity for improved tone and tuning stability), redesigned hammers with premium underfelt (wider tonal contrast and durability in tone).
The B3 and P121 are within your budget and are both significantly better than an early U1, plus they are brand new pianos. You need to compare them to know which you are most happy with in touch and tone as well as appearance.
The Yamaha B3 has a laminated spruce soundboard. Laminated spruce is 3 plies of soundboard-grade spruce bonded together. Its is more cost-effective, more durable and stable with fluctuations in humidity - and it is well-engineered to produce a consistently good tone.
The Yamaha P121 has a solid Bavarian spruce 'Strunz' soundboard. This does improve the clarity and sustain. More top and bottom end to the tone.
Both models are built by Yamaha in their Jakarta, Indonesia factory. 100% YAMAHA - not a joint venture or 3rd party. Excellent build, finishing and control. All the other components and structure of the pianos are identical/same to the current Yamaha U1.
Go back to the refurb. U1 and listen to the bass. It will be tubby / dull and have little sustain and clarity. Go straight to a B3. I guarantee the B3 will be much much better. Listen to the tenor (middle). The refurb. U1 will have a more percussive tone and again, less sustain. The B3 will have more depth, colour and sustain - and more dynamic range. But if your budget is £5k - then you should be comparing these with the even better P121.
There are many decent pianos available new, within your budget. You could spend a year trying them all out!
I could guess you will be told that the earlier U1 is better than a new one, or perhaps better because it is played in, or better because Yamaha made fewer pianos then and to a higher standard with more time spent... or that the new U1 is almost identical and virtually nothing has changed bar the soft-close fall, or perhaps you are told that the B3 is a cheap/budget piano and the plywood soundboard won't last etc. These are what I have heard from some customers who have been mis-advised and all of these common words of advice are technically incorrect and certainly misleading. Excuse the pun, but these industry is in need of regulation.
Topic: Yamaha U1 and Petrof S1