Is it worth asking The National Archives?
Yes. However, we've already done that! Generally, TNA maps are misfiled and tucked away elsewhere.
The NLS has the pair of 1932, a north sheet of 1936 (that they had misfiled as a second copy of 1932 before I got there!), and pairs of 1946 and 1956 sheets, then 1964 onwards.
Ten Mile Road Maps are awkward - we don't even know for sure when they were printed! As an example, I own a copy of the 1948 south sheet (which doesn't appear to have a north sheet equivalent), and it's actually on SABRE Maps mislabelled as the 1946 sheet as absolutely everything (other than the relatively minor numbering changes) on the map actually states "Correct to 1946" etc, and it's just the different print code that gives it away.
There will only really be academic libraries that might still have copies, and beyond that private collections. The National Library of Wales (or whatever it's called in Aberystwyth) is a possibility that we haven't explored.