Hi, look the following story might sound barmy but I had the 7k error message on my 2005 Gen 1 a while back in 2009. I asked my technician to sort it out. He serviced it. Problem still there. It always happened after a spell at highish speed, typically 80mph for say 5 minutes. Slow down for the roundabout and there is the error message. So, with no 'bend' apparent in the valve, my technician reamed the bushes just a touch as we reckoned it must be binding following high speed by getting hot and sticking. Problem still there after reaming!
Well, in 2010 I took it for its MOT to a workshop I hadn't used before. Man there said..."we've had a few of these with cracked exhaust just at the back of the collector box/exup, bl**dy expensive to repair and getting the pipes off the cylinder heads is a b*gg*r. What you need to do is make sure you tighten the clamp bolt here at the join of the pipe to the box regularly, as it seems to loosen and allow the exhaust to vibrate and crack the pipe". So he got his spanner while waiting for the online authorisation from the trolls at DVLA, and swung it onto the nut. Sure enough it was loose. Did it up tight, not too tight to strip it though you understand . Lo, and behold the exup error code never re-appeared. Couldn't believe it. Told my technician who also was astonished. "learn something new every day!" said he.
I checked it from time to time and sure enough it works loose. So...if indeed there was air creeping in on the overrun from high speed or in some conditions, then it's possible the temperature was reaching superheat and causing the exup to stick. Just a theory.
Hope this ramble saves someone having either cracked pipe or exup aggro.
John