Well, the Lokprogrammer wasn't up to much, for some reason Widows 10 isn't recognising it as a com port.
I did get DecoderPro working with my SPROG so that made things a lot easier.
Firstly, doing the uncoupling sequence from a hand throttle gave the same results so its unlikely to be anything to do with TC.
After reading the Loksound decoder with DecoderPro i managed to strip the uncoupling sound from the uncoupling function.
Hey presto, it now works perfectly.
Allocating the uncoupling sound to another Function key and activating it just before the uncoupling function messes it up again.
Strange.