Dear Rick and all you helpful others
The problem has gone away, months after it started, as mysterioulsy as it arose!
We had bashed on printing 4x6 with a white stripe and using a guillotine to trim them down. I delayed posting this for several days in case it came back but have now printed dozens of 4x6 as perfectly as it used to. It all went right when my wife put a fresh stack of paper in the printer. Perhaps she has a special touch but I wonder if a roller or something had got jammed or sticky and had finally sorted itself out. I had not changed any computer settings. I'd previously tried cleaning the printer rollers by running lots of blank A4 through it - without apparent effect then.
Thanks for listening, everybody. If anyone else gets the problem then
Matthew
