Hi,
We used londongreencycles -
http://www.londongreencycles.co.uk for looking after the Christiania I bought a couple of years back on eBay - set up saved searches on there, pre loved. London Green Cycles are brilliant and will let you test drive different ones to see which you are most comfortable with.
I recently traded in our two child one for the slightly bigger box, feels very flash having a new bike. They have me a great deal.
I love our cargo bikes and don't really use any other transport now - round commute is now about 16 - 20 miles a day with 3 kids (4,2,6 months).