Further update - I had the car serviced last week and while it was in got the injectors tested. I don't know what the numbers mean but 1 2 3 5 and 6 were pretty constant but no.4 was a lot higher reading, so that one's getting replaced next week. I had then replace the plugs aswell and my range on dropping the car off was 165-odd (having driven it there nice and slow), and when I drove it home it went up to 300 - so some improvement seen already.
Fingers crossed the injector replacement will make it all OK again, and the best bit is the warranty are covering the cost of the injector and diagnosis so all in all not too painful!