Hi all,
thanks very much for the replies.
Spent the evening fiddling around with them, following the replies.
@Richard, I put in a double 'D' battery holder, as I found out along the grapevine about the original batteries being 1.5V each.
@Dave, Indeed, I swapped them as a whole, and it was just simply quiet/low volume. Thanks for the description of how to gain access to the transmitter, and of the granules inside it (Simple when one knows how! lol). I gave both transmitters a gentle tap, and the granules sound much more free, as well as a gentle clean of the contacts (Bit grubby, like the rest of the phone was).
@McMurdo, I also gave the switches an inspection. only one wire had had it, and I replaced, but are good contacts.
I wired the phones up again, tested them, and the sound in both phones have much more clarity, and is no longer quiet.
I do WW2 exhibitions showing some of my military collection. I pride all of my collection with 'fully working' artefacts.
Many thanks for the help - an excellent and helpful resource!
Best regards,
Simon