04 1.6 uk focus rpm issue

Last night my focus started a bit of strange behavior. If I rev the engine then back off, the revs don't settle smoothly. The rpm sticks at about 1800
rpm for a few seconds then plummets to about 500rpm before settling at about 800 rpm. The car drives perfectly and I haven't noticed any lack of power and it doesn't cut out or splutter at all - it's just kind of annoying when it holds the rpm when I'm sitting at traffic lights.
The car has only covered 14k miles from new and is actually booked in for it's 2nd service next week but I'm looking for a heads up as to what might be up.
BTW I had a VW Polo exhibit the same trait a few years ago - It turned out to be the throttle cable catching on a floor mat. I've checked the cable on the focus and this isn't the problem this time.

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