Your new 12 is the sister-shop to my boat - 2935. both were built by Dave Proctor in Whitstable, if fact he made three Cheshire Cats. I haven't sailed my boat much since the 90's but last year I decided that as she was 40 years old in 2016 she deserved dragging out of the shed and putting back in commision. In fact in a moment of craziness I have entered for Burton Week. Unfortunately there isn''t very much activity for 12's now in the London area so we haven't been out yet this year.
In answer to your question re sails it sounds like someone has picked up some used-sails for your boat. If they were made for a carbon rig you may need to change the luff-curve so it would be an opportunity to get the luff-rope matched up to the mast at the same time. So a trip to a friendly sail maker looks the best bet.
Glad to hear you've another 12. I think my boat has the same type of mast ram but it's fitted above deck, I think the problem may be the control rope chafing / jambing. From memory the first owner of that boat didn't sail a great deal so she has probably had a relatively easy life; certainly the Paper Dart was always a tiny bit quicker on the river than the Cat in the very light stuff, the tables turned when the breeze came on. I can tell you a bit of interesting history about that boat.
Let me know if you need a hand with the mast ram, you should have my number (Pip will if you don't).
I'm looking to make a rudder stock / tiller assembly, I'm thinking of using carbon tube for the tiller but I've never bought any before so, what spec should it be and who might supply it? I'm also wondering if something like a broken spinnaker pole or similar might work (I suspect it won't be strong enough). Any suggestions / advice would be welcome.
Getting the boat up you will have pulled centerboard right down, once upright with all the sails flapping and chaos going on its easy to forget to pull it up before you try and sail off - the boat won't want to bare away with the centerboard right down. As you do bear away and pick up speed move right back, the crew too but not too soon.
International marine paints from any of the big chandlers, I have had success with paint from the DIY shops but make sure you buy oil-based not water based paint.
Interesting. I think that something like this actually happened once, some years ago ... I recall hearing a story (I think it was Janet Bloor who told me) about a phone call one night from the coast guard. A capsized National 12 was found in the channel and the coast guard patched the phone call through to the SAR pilot... Maybe Janet could add some more detail, if I am right that is! Dave C