As above, I only do between 3k and 4k miles a year in my 2003 TD6, how often should I do an oil and filter change to keep her happy?

If it's short journeys change it every year, however if they are long(ish) journeys then every two years

You can never change the oil too often

I always service all my vehicles just before the MOT so they all get at least one per year irrespective of the miles driven, also gives you a chance to do a pre MOT check

Once a year for me.
I cover 4-6K a year.