Thank you all for such very helpful advice and encouragement. I have looked at all the links and photographs, which have answered a good few queries.
Yes, it is inevitably a shelf set-up made of 4" wide x 2" thick tanalised board, to go with the chunky ambience inside the Midland Railway's iron tank. Brackets will be at 4 foot centres in fact - I can drill and plug into the sealant between the 4 foot long cast iron tank plates. The sealant is amazing stuff. Obviously applied in a semi solid state originally but it sets like cast iron itself. I am told it was a concoction of putty, bitumen, powdered coke, iron turnings, sulphuric? acid and one of the ammonium compounds. I guess the idea was that it rusted, expanded, then set solid. Anyway the tank did not leak one drop.
I am grateful for the roofing felt idea and shall ensure it is stuck down thoroughly. Intrigued by the soldered wire connectors between the rail sections. I was thinking of drop wires to cables below the deck. Does that sound like a good idea?
The entire shelf is against the continuous tank perimeter, inside which is the roof room and walkways so I have no complication of an access flap.
Weather permitting I hope to make a start this week. I have an existing Blog settlestationwatertower.blogspot.com so I shall be posting pictures of the progress there. The real expertise clearly resides in this forum so I shall rely on you guys to keep me on the paths of goodness and righteousness and to prevent me from making too big a public fool of myself.
In passing, we are on TV in the USA in the next month or two. A programme called "You Live in What?" This is quite separate from Restoration Man. A film crew from Dallas Texas came specially. We have our fingers crossed that they will be gentle with us.
Mark Rand