Hi Eddie Simpson, I have done some tests this morning and first of all found that my first multi-meter was faulty (now changed). When the bike is running at idle it reads 0 on the ammeter. when revved up it increases to just above 0. with the lights on, it drops just below 0 and back to around 0 when revved (headlight gets brighter when revved up).
With the engine switched off I got a reading of 12.25 volts across the battery terminals. when I switched the lights on for around 20 secs the voltage dropped to around 11.25 volts. When I started and ran the engine for a couple of mins then switch it off, it jumps up to around 12.80v.
I also found that when running i'm getting 14.30v at idle which increases to 16v at 3000 rpm, when connected across the battery terminals.
I was thinking rectifier, but then thought it would of blown the bulbs by now.