Ford fiesta 04plate - speed idle sensor problem?

It does sound like the idle solenoid may be malfunctioning. But to be on safe side, I would take the veh to be checked. It could be a problem with fuel pressure too. The fact is, it could be a lot of things. To just start replacing parts could be a lot more expensive than taking it to someone reputable to find cause of problem and get it fixed right the first time. As for the cost, it really depends on who you take it to, a ford dealer would be my first option, second would be a Commercial shop with good reputation. Yes dealers are more expensive, but they can find the problem in a hour or less, where it make take bubba half a day to find the problem. where they have a cheaper labor rate per hour, it may take them more hours to find the problem. Hope this answer helps, and good luck.