There's definitely not a cassette in there already. The display is blank, so I don't know if it's asking for the code, but I've tried to input it several times without success. I've seen a head unit on Ebay for £80, but I'm not certain that the fault is there and not elsewhere in the system.
It's a flat fee of £98 I think. They do have a site on the Bay.Many thanks. I can't repair it myself - unless it's a VERY simple fault, like a loose connection. I'll ring Clarion for a price. I've also been recommended to try Mapler Audio in Bath which isn't too far from me.