Been a while since ive worked on an SS machine. But... the GI is straight from the transformer yes? Its the 6.5VAC line that goes through TP4 and fuse F5 ( 20amps).
If the potential difference between the two sides of the transformer output is only 3V then I would test both sides of the 6.5VAC line AT the transformer and read the voltage there. If good.. move along to the fuse etc.
If bad then you have either a drop at the high voltage or an increase at the low ( AC ground 0V). If you measure both of these with reference to ground then you may find a reading such as : 9.5V at high and 3V at AC ground.. gives you a potential difference of 6.5VAC.
Often a stray whisper of another cct is touching part of the GI and throwing the PD out.

Thanks Gav,
I'll look into it again tomorrow.

Its amazing how simple something like this seems once you've found the problem, its finding it the problem that the trouble 

Its just a matter of elimination 

Good luck & hope you find it 

Yep, so oftern the way that the once you know what the problem is then the fix is simple. It's just identifying the problem that's the issue, particularly with my limited tech skills.      Thank god for AP and friends who help out.