OK, having read this thread on storage I get some ideas about issues now and am starting to feel it is best I "loosen up", consider using storage space at my place of work for prolonged spells off road, and learn how to remove the bottom brackets and see how I get along with doing that (it is likely to be something I only do once I imagine).
I did take advice about where to put drain holes (at each bottom bracket) if I make them and I am still unsure if it is a good idea to do this and have no idea if, having done so, I should use tape squares to cover the holes left from manufacture on the rear dropouts. The advice to grease heavily the seatposts was sound. There is no corrosion of oxidising to speak of there - that is one of the few areas I have been busy with since we got a thud buster for stoker and prior to that considered swapping posts (but I found mine was too long and had to swap back).
I am really sad the bike is going to spend most of it's life out doors on the railings, but I am sure you'll understand if that's what it takes to keep stoker happy then it is for the best - an unhappy stoker means no (tandem) rides.
I think this is going to be a noob question now - but why should the drain holes be at the bottom brackets? I imagined I would just need one and that it could be more or less anywhere along the bottom tube....