It's most likely the EGR valve. You have one of two options - The most sensible one is to buy a new EGR (NOT from Ford, as they charge a shed load) Look on eBay - Just make sure you get the right one. Or... As it's a 56 plate it's almost certainly a Euro4, so you can blank off the EGR but it will bring on the management light - so while it might 'cure' the issue, you're still left with the light on.

Swapping an EGR is a very easy job too, if you've got basic tools you can do it yourself.

It 'could' also be a problem with the vac system (Vac solenoid) but the EGR is more likely to be at fault.