I've been looking too, Robin ... Severn Rail Bridge absent, so map dated after 25.10.1960; Ruthin shown as having purely the stub service, so before 4.4.1962. In spite of label saying circa 1959. Agrees with how Melton Contable is shown - as a branch from Sheringham, 1959 to 1964.
It's not that simple Graham - in fact the map is quite a bit of a mess!
As you say, the Severn Bridge was redesigned in October 1960, but initially it was planned to repair it. The link below takes you to a scan of he timetables for 1959 and 1964, the latter one still advertising the line as temporarily closed.
https://www.flickr.com/photos/93122458@N08/28605810524/You will also notice that the section between Berkeley Road and Sharpness remained open until final closure in 1964 so, if the absence of the Severn Bridge is taken as proof that the map was produced post-October 1960, it still doesn't explain the Berkeley Road to Sharpness omission. And especially when, not too far away:-
Coaley Junction to Dursley showing on the map, closed in September 1962
Ross on Wye Monmouth Chepstow - not showing on the map, closed in 1959
Chippenham to Calne not showing on the map, closed in 1965
MSWJ Cheltenham to Andover showing on the map, closed in October 1961
There are plenty of other examples that simply do not make chronological sense.