Nissan Qashqai - the +2 models gives you an extra two seats in the boot, at the expense of luggage space, when you need them.
My manager has the 1.5D and is getting 47mpg in normal driving; I've managed 51mpg on mostly motorway journeys in it (both numbers from the trip computer, so probably less in reality). It surprised me how frugal it was, and the engine never feels laboured, even up to 80-85 on the motorway.
She (my manager) also has a dog, although has transported two dogs in the boot without problem.
At your price point you'll have loads to choose from in the age range specified. The Ntec version has built-in satnav and a reversing camera which is really handy.
We have the same list as you, except for 2 dogs rather than one.
We went for a VW Touran and it's been fantastic. Its diesel and has good MPG. Only own side is nearly £200 for car tax.
Love the driving position, helps kids see so much more as we drive too.
7 seats has been very useful, the way the last 2 fit completely into the boot means if you don't need them they are not in the way. If you discover while you are out you need an extra seat you don't have to rush home to get them.
We're tossing up between estate shapes and the smaller crossovers as I like the high driving position. But, are they very uneconomical? Some background:
2 kids 1 dog Spending up to 15k, prob 1-3 years old Decent boot, but I don't want it to feel too long Economical to run, as far as poss for a reasonable sized car 7 seats would be a plus, but not vital Needs to be massively reliable, I'm clueless Needs to be a work horse