Yes, I've read a bit more about this kind of thing now.
There seem to be two different image problems. One is the IQ problem which is difficult to completely get rid of as the IQ correction in SDR# isn't quite doing the job. Though it is pulling the spike down quite a lot I still get a significant spike and weak mirrored images do make an appearance in empty frequencies or on top of weak real signals if there is a strong signal in the 1.5MHz window.
The other seems to be out of band signals coming into play from approximately 6MHz down, and above 320MHz up to roughly 380 MHz there are FM Broadcast images, DAB images, all sorts all on top of one another. The gap then comes into play and the band is suddenly clean when you click over to 400MHz.
Reason: No reason