The photos make the piece of wood look worse than it actually is, and the loads upon the hinge screws will be very small as the table is supported fore and aft and will only be carrying the odd plate of pasta and bottle of wine. So while I search for a nicer piece of wood I'm going to continue with the ply do some more tests of how well it holds screws. I don't fear water damage really as it will always be in the dry and will be well varnished and edged with hardwood trim. I have to keep reminding myself that it is replacing an old table made from 9mm ply (probably better quality) which is still perfectly good - just too small and too fiddly to erect. The seats upon which we will sit are also made of the same stuff.
On the subject of wood, visits to local suppliers have confirmed my suspicion that quality is going down fast. I haven't seen a "good" piece of ply anywhere, and the hardwood trim is hard in name only. I suppose its all to do with protecting endangered forests so I mustn't complain.
When I bought my best guitar 15 years ago the difference in cost between an Indian rosewood back and sides and Brazilian rosewood was £300, and good Indian rosewood isn't cheap! I can only speculate about the difference would be today. It would have to be very old Brazilian, because they haven't been allowed to cut it for years.
If you want to see some lovely wood, anoraks might like to follow this link to the great Stefan Sobell who made my guitar and click on his page about wood and the efforts he goes to to find it. Then you can look at the other pages about his instruments and drool.