Thanks craze, I'll investigate further ...some more tools to buy!

As part of registering the car I had to quieten it substantially, so instead of the Magnaflow straight through mufflers it came with from the US it now has a muffler and resonator on each side... from further research (on a Mustang site) it looks like extra back pressure can cause very rich mixture so I should at least give it a tune up... I'll also pull out the other bank of plugs to see how they look.