I know how you're feeling Shaun, I've got two vehicles here at the moment that have severely tested my confidence in diagnostics, one you will see on the forum is a simple Agila 1.0cc with an EGR fault and another Vauxhall Astra 1.9cdti with a swirl motor fault that were both tested before replacement and (genuine) parts were fitted and they still have problems! We won't go into the hours that I've spent, but to say I'm not making any money out of them is an understatement!