On Jun 20, 3:34*pm, "Steve" wrote:
Internal door, solid wood panelled job, so I'm not going to be doing any
woodwork hollowing out the frame to take a closing mechanism.

Current fastening seems to be a round handle on each side that is twisted to
open - bar that passes through the door, with the mechanism screwed to the
room side of the door - I don't know what this is called, but there must be
a name for it in the trade. Does the job adequately, but additional
functionality is now required.

Would like to replace this with one that is lockable from outside the room -
any suggestions?

ISounds like a rim lock - but that would normally have a key-operated
locking bolt with a keyhole on the outside too...If it isn't then you
could fit one - the only woodwork would be to drill and cut a keyhole.
If you have an old rim lock which has lost it's key then a locksmith
will make a new one to fit.