1.8 spi (passat)

i know this isn't a polo but just wondering if any on has any idea's because its baffling me.

On tick over it runs a little lumpy and has a slight missfire but seems to clear when in the rev range. I took each ht lead off one at a time while it was ticking over and it effected running on every one but number 4 (plug at dizzy end). I swapped plugs and leads around to see if this moved the problem but it was still happening on plug 4. I also tried a different dizzw cap which made no difference.

About 6 weeks ago towards the end of a 80ish miles of motor way driving it started running like shit, new plugs and leads were fitted and it seemed to sort every thing but this idle problem.

It has just fail the mot on emmisions which i'm assuming is tied in with this issue. After its been revved the revs hang a bit before dropping back to the 1k tick over.

If any one has any idea what this could be i'd be very greatfull for any advice before it gets given up on and scrapped