Thanks mate. I do have a code reader but it's quite a simple device and although it noted there was a fault, it didn't pick up code that's on the list.
I use a garage who are into this kind of thing so I'll get them to check the code before I delete it just in case there is something more in depth.
I'm reasonably sure its a case of a rotten intercooler. It looks pretty corroded and being it's right at the front od the radiator stack it probably gets all the dirt / grit and salt thrown at it.
Bit of a relief it's the intercooler to be honest...at least it's a home fix and nothing else was apparent.