Hi Henry, don't want to get you too worried but when I was looking to buy a 155 I view'ed about 10 cars, the only one I saw with a clock that had a display that worked was the one I bought. Looks like it may be a common fault.
Sounds like a Liquid Crystal Display that's knackered. They don't like water, perhaps a bit of moisture has got in there at some point. Don't know whether they just deteriorate with age? Either way, I think you're looking at a new clock mate