Hi Polyglot,
OK sir, I will take your advice and try the 6N138/9. (I'd better get a couple in case I screw up!)
No I wasn't thinking of driving the neon directly (!). Many of the systems I work with have dry contact on/off outputs (relays). It makes it pretty easy to hook up whatever you want - mains, 24v, 5v and gives you a decent current capability. I know in this case it looks like overkill, but it would both future proof it and be familiar to anyone else who has to work on it. I was thinking of dropping in a transistor driven small DPDT relay, one pole for the output and one for the existing neon.
Many thanks for your help and advice. Very much appreciated.