Quick update from the UT61E department. I decided to try and solve the serial adapter being useless with USB/serial converters. I found the schematic online for the UT61E that covers this:
I think they are a little optimistic to get +/-12v out of a cheap USB/serial adapter, or are they? Hooked it up to my DMM, hooked up a small electrolytic cap and watched the voltage climb to a not too shabby +/-12v on the mark. Hmm. So I knocked up a quick RS232 male/female patch and plugged the adapter in and we're down to +5v/-3v under load which isn't enough to bias the transistors in this into anything usable. Which is why they don't work!
I snipped the RTS and DTS lines on the patch and hooked up my bench supply to provide +/-12v and it now works.
Modified schematic of interface:
Now it's a bit much to drag 9Kg of bench power supply around with this so I'm knocking up a small inline adapter with a couple of MC34063 ICs and a generic wall wart socket in it which will do the job.
I suspect the root cause of this is the USB/serial adapter being rubbish (it was only £2!) more than anything else.