something a little sporty (so i can have fun at the weekends)
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Does not compute. Depending on how large you want your fuel bills to be, B8 shape S4 Avant would feature VERY highly on my list. As would a BMW 335D/I Touring. Both a bit smaller than Mondeos etc but on another planet in terms of build quality and ‘sporty to drive’-ness. Might be a squeeze with 3 kids across the back too.
I think for 3 kids in the back you need to be looking at big estates and people carriers (so kiss goodbye to something sporty for the weekend). A6s, Mondeos, Passats. In fact, if you can find one, a Passat R36 might tick the boxes and you should get one with at least £5k change from your budget for a perfect one.
Otherwise, family box on wheels like an SMax/Touran etc etc and a mk1 MX 5 for the weekend + a load of change.