Intermittent Power Loss - No Codes

[Astranought]

Hi. The car is a 2010 1.6 Petrol Exclusiv 113 - 64000 miles. From cold it runs perfectly but after a few miles it is intermittently loses power under load  - going uphill or accelerating hardish. It feels more like a general reduction of power than loss of individual cylinder(s) but I'm not sure on that. No codes are displayed while the fault is occurring and there are none stored, according to my local garage. I've read elsewhere on the forum of similar problems resolved by a fuel filter change and the garage mechanic suggested that may be the issue but then discovered that the fuel filter isn't a replacement part on this model. It's integral to the fuel tank? 
I have changed the plugs but not the coil pack as it looks like it has been changed before - probably by the previous owner trying to resolve the same problem. But wouldn't a faulty coil pack yield a code?
I'm on the cusp of selling the car at a substantial loss because of this but I wondered if anyone had any thoughts?
Thanks in anticipation.