My boy had graduated to a booster seat but having seen a rather shocking video crash testing car seats we decided to get him a larger car seat with better side protection for his head.
See video:-
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Child car seats | Crash test video
This is the one we bought, in fact we bought two one for the Mini and one for the Jag. Bad news is it doesn't fit the Triumph
Carseat Brand: Britax
Carseat Name: High Liner
Attachment method: Seatbelt
Childs weight: about 3 stone
Childs age: 6 years
Carseat weight range: 15KG -36KG
MINI Type: 1st Gen Cooper
Body Type: Convertible
MINI build date: 2004
Interior trim: Leather Sports