Whilst wating for some ferrite beads to arrive, I decided to dig a little deeper into my circuit, disconnecting the outputs of the phase splitter from the two output devices, and output noise virtually disappears - just bit of mains hum creeping in due to the loss of the relatively low impedance drive to the gates.
The prototype uses a pair of VN10LMs, hand selected for reasonable matching of the gate threshold voltages. These devices were chosen for no other reason than they have an adequate Vds rating and are readily avalable.
I think I will get much better results if I replace the mosfets with jfets. I will however be fitting the ferrite beads as a precaution.
(Sorry to have to use Google to post on here, but it's the only newsgroup reader I've found that works with TalkTalk).
Regards
Ted Wilson