Hi John thank you for your reply and indeed Kudos to the Gurus. From holding Ammeters the wrong way round to now discussing Gate voltages and Drain currents.... :-)
OK now I get it. Use current as the measure and not just voltage.I was reading about break down voltages at 3-4 volts for IRF510's but glad to know it is ok to adjust to 200ma aka 3.60&3.99V and just leave it be!
I managed to solder in a couple of resistors at R60&63(?) to stop me having to bellow to get a response from the audio.
All in all a great education, thank you.