Thanks for all the replies so far. I've recently been trying to replace short car journeys with a bike. My hardtail MTB (Pinnacle Iroko) is great for bombing around the woods or trail centres but I rode it to the doctors surgery and back the other day and it was really hard work (very long uphill return). Wrong geometry, wrong tyres, great bike but the wrong bike for the job.
My long term plan is to get a bike for trips to the shops, errands, train station, "relaxed, comfortable, flat (mostly), practical urban journeys" as wafter wrote. Hence my interest in a dutch style bike . I realise that dutch bikes work particularly well in the Netherlands because of its terrain and the fantastic cycling infrastructure they have and that might not translate to riding round Northampton, UK. I'd prefer a more upright riding position (I had open heart surgery this year, I prefer not to place to much weight on my arms/chest), hub gears would be nice but I'm ok with a derailleur if the bike is right (at least 3 gears, preferably more - my current bike is 1x10, great off road but not so great on). I say hub for the hopefully reduced maintenance. The ability to fit a cafe/wheel lock would be a bonus, same with a pannier rack, kickstand, mud guards and chain guard. New, used or vintage, I don't mind. Used is a plus as it probably wouldn't be as knickable. Budget would start around £400 but like I said, I'm not in a massive hurry and the budget might rise for the right bike.
True Dutch or Hybrid, I don't mind but I'd prefer more of a step through frame. I suppose I'm looking for something with the best aspects of a Dutch bike rather than a true Dutch bike.
The 531 Mixte frames look fascinating (thanks DCBassman), I wasn't even aware they existed. With a slightly taller stem and the right bars I reckon one could be made to have a more upright position. So many bike rabbit holes to disappear down when you join a bike forum... I can see this being an expensive pastime...