[johnny_quattro]

It's the simple things in life that count.

Battery terminals were level with the top of the battery posts but the voltage was still low with the engine running. My positive battery terminal (and the post) were warming up rather nicely which proved in my non-electrician's mind that there was some resistance causing the problem. I went to the effort of checking for parasitic drain whilst I was there, even though that wasn't the root cause of the problem. Nothing out of the ordinary when I checked the mA draw on the multimeter.

Took both terminals off the battery and inspected the battery posts. I noticed that there was a vertical ridge on both terminals which you could feel with your finger, let alone a nail. I suspect that this may have been down to the local grease monkey over-tightening the terminal - this may have been due to the work done where the local garage replaced my fuel pump recently. I managed to remove the ridge that was present on both posts and then applied some copper grease to the terminals.

End result - was able to start the car without my jump pack for the first time in a week or so. I checked the voltage and was now seeing 13.6V with the amp/stereo pounding away. Checked the posts after 10-15 mins of idling, and both terminals were 'cold'. I've just been on a good run in the car, with some spirited driving as they say. Famous last words and all that, but I think it's now sorted. Motto of the story - always double-check connections even if the terminal connections appear to be well secured to the battery posts.