Richard it seems you are not going to get any definitive replies, so I'll put my head on the block to try to move things on for you.
There seem to be quite a number of these SRL14L plugs around & you can find many references in forums where people seem to have used them successfully in Rovers.
However I have never seen a formal statement in any parts books for Rover or Humber that they can or should be fitted. So I don't know for what vehicle they were primarily used.
SRL14X was fitted to Rovers 10 & 11
SRL14PC was originally fitted to Humbers as Plug, sparking, No.1 Mk 1
With Lodge plugs I don't know the significance of the suffixes X, PC & L.
This afternoon I had a rummage to see if I had a SRL14L, but could only find a SRL14PC
To me it looks very likely that it would do. But if I had one in my hand I would compare it with the more usual plugs used before fitting it. But I think certainly at this stage of just getting running at all it would be reasonable try it.
If you are in a real quandry & that is the only type advertised, I can sandblast a used RSN13P & post it to you.