Quick update. I moved the Comar receiver out of the loft and back into the office, with a short USB lead that has ferrites on both ends. Then put a clip-on ferrite ring around the antenna lead, and positioned as far away from the Radarbox as possible, and it now seems to have minimal affect on Radarbox reception :)
I also received the marine band antenna yesterday from Maplins, but I've had to return it because it didn't included the 5m lead and BNC convertor :(
So, still running on my discone, and I'm getting approx. 15 miles range even on that.