Yep, found the problem(s). A small vacuum hose had come off the throttle body - it was that which was throwing the 'multiple cylinder' misfire code. Once that was re attached, I was left with a single cylinder misfire on number 3. Swapped the plug and the coil pack around with number 1 and the fault stayed on 3. Swapped the injectors round and the fault moved with it to number 1 cylinder, proving that the injector on number 3 cylinder was faulty - it was breaking down when it got hot. New OEM injector from EBay was £35, put it in and it's now running perfectly.
All that said, multiple cylinder misfires can be caused by a whole basket of things.
Cheers and good luck with finding the problem
Paul