Bryan
Finally some success.
I have managed to get the M15 working on an external supply.
The lead was from Amazon. A 4.0mm x 1.7mm 2A male plug DC adapter.
And a battery from Screwfix, product code 20080 (2.1A sealed lead acid battery).
Soldered spade ends to the wire. All good.
Sadly Screwfix don’t sell an IP67 rated box that’s ‘just right’ size. I can tell you they sell one that’s not quite ‘just right’ size!!!! So still looking for that bit. So far so good. Nick