Hi npl, I see why you had issues finding anything! The only thing that I could find was this quote from Britax:
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Q. My child weighs over 36 kg. Does it still have to use a child seat?
A. The legal limit for the use of a child seat is either 1.35m (1.5m in Ireland) or 12 years of age. Child seats for older children are restricted to 36kg for test reasons.
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http://www.britax.co.uk/customer-service/faq
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I think they are saying that because the UK/EU only requires the seats to be tested up to 36 kilos, they only test them to that standard. But they can (and should) still be used for longer. In your situation I'd try and buy one of the higher-end seats (Graco/Britax/Maxi Cosi). Check the manufacturers website for fit and dimensions, I have found errors on the Mothercare website before.
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I've also encountered differences in seat ratings before, for instance the UK Nautilus is harnessed to 18 kilos, where the US Nautilus is harnessed to 25(26?) kilos. Very confusing!
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I hope this helps a little! The only other thing I can recommend is The 'In Car Safety Centre'. They are pretty expensive compared to Mothercare/Kiddicare, but they might be able to give you some advice if you email them directly. Their website is
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http://www.incarsafetycentre.co.uk/about.html
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Hope you have a fantastic holiday with your family!
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Edited to add: I forgot to answer 'What would you do?' - My eldest is just shy of three - so I'm not quite in the same situation as you. I would personally buy a booster rated to 36 kilos and use it. And make sure it's adjusted and installed as perfectly as possible. To me, personally, the benefits of correct belt positioning outweigh the fact that the seat hasn't been tested over 36 kilos. My double stroller is only rated to 15 kilos per child, yet I happily put my 17 kilo son in it. I'm pretty sure it could hold my weight and I'm a fair bit more than 15 kilos ;) I know, I know, it's a stroller not a car, my point being, that the seat doesn't magically lose all effectiveness if the user is over the weight limit. :)