Fiesta (2012) Mk7 revs erratic & cutting out

Hi, my Fez started cutting out today...

Yesterday the revs were slightly lower than normal, just above the point where the car would stall, but other than that it was fine when driving.

This morning the same thing, low revs but driving ok.

Then this afternoon I started the car and the rev counter was fluctuating between about 500rpm and 2000rpm, slowly going up and down. At tickover I was having to rev slightly to stop the car from stalling.

Driving the car it was doing the same, rev counter fluctuating up and down and the engine sounded as though its running rough and whenever I stopped, at lights, or at a junction, or in traffic, the revs would drop and the car would cut out.

I limped home from work In Croydon, to Bromley and managed to get to the main Ford dealers, they said they will look at it later or in the morning, so left it with them.

Any one have any ideas what this could be?

I had a quick lok under the bonnet myself to see if I coudl see anything obvious, but didn't note anything, but then again, I'm no expert mechanic...    8-(

My Fez is a 2012 1600cc Titanium model with 35k miles and its serviced every year and has had no problems to date and I don't abuse it or drive like a *****