Thanks, I had noted that series of wagons had steel floors, over 500 of them, but didn't realize that they were subsequently converted to double beets. I had previously assumed that the height of the 11817-14672 series of Bulleid corrugated opens had been doubled but these were on a 10' wheelbase and had a capacity of 12 tons. Now that I delve through 3rd edition of locomotives and rolling stock of IR & NIR by a certain author, I see that 166 of them were converted in 1985 to the 28501-28665 series of double beets on the 12' 25436- flat chassis, two bodies from the 11817- series wagons with a capacity of 19 tons. I presume the additional length in the body would have allowed additional length to brace the double height body appropriately.
Mayner's flats look fantastic but came out just before I came back in to modeling so they didn't make it onto my radar. I'm not sure they're available anymore