Iridium Spark Plugs in a Mk1 MGF 1.8 MPI

Don't do it!
Platinum plugs work best.

Now I have 4 Iridium sinkers for next time I go fishing

Its interesting to hear your experience of the 2 sets of plugs back to back, that they made such a vast difference. Were the iridium plugs in when you got the car hence might have been old or were they a new set that you put in ?

What was the difference between one type & the next that made changing them an imperative.
Even the falling off in performance is difficult to detect & providing there isn’t a catastrophic failure is there a notable & sudden improvement in all departments.
I change plugs as part of a service & some service intervals are less critical than others, cam belt before plugs if the two are due at the same time.
Any arguments against that assessment?
M