Xack, thanks for the suggestion. I don't have switches etc at my disposal right now but I like the approach.
After running new wires to the amp with solid earth etc, nothing has changed. So I decided to install a relay for the ACC power to the fuse box, triggered by the ignition instead of a straight feed from the ignition switch. PROBLEM SOLVED.
In hindsight it wasn't a great idea to draw power for all accessories through the ignition switch and I still don't know where the leak is/was but after two days messing around I just don't care.
Thanks for all the suggestions - believe me it helps just knowing people are out there feeling the pain!
Time for a beer
