Think you could be right RS2K and I think I may have found the problem.
I managed to get the wheel liner off at the weekend and have a good look at the aerial. The aerial and the lead coming from it looked to be in perfect nick, but I found that the point where the car aerial lead was joined to the cable from the back of the radio were joined together using black masking tape!!
When I removed the tape and seperated the wires there was no change in reception on the radio. It seems the aeial had never been working in the first place.
I had thought that the cable running from the the aerial would have connected directly into the back of the radio. The end of the cable looks like a 3/4 inch pin, with a thickness less than that of a golf tee. However, the aerial attachment point on the back of the radio won't accept this. It looks more like a TV cable point. I might be answering my own question here but would I need some sort of adapter. If so, does anyone know what type I need and where I might get one. (96 Seat Ibiza).