I have a 2003 Vectra, which suffered from severe radio interference on medium wave, but only when the car was moving. There are several newsgroup articles describing this problem which also affects Astras, but with no definite solution. I took it to the dealer several times, and they first reseated the cables, then replaced the radio (under warranty), then reseated the cables again, and each time it was OK for a day or two, then the problem returned.
Finally, I persuaded them to talk to Vauxhall technical support, and they were advised to fit a suppressor to one of the cables (sorry, they couldn't tell me which one), and this has cured the problem.