I get my Lindor by the kilo...
If the contents are indeed 400g, then Waitrose currently have a deal on the 200g boxes at £4.00. Get two of those and save yourself £2.00. Tin will likely go in the bin anyway!
There doesn't appear to be any photos showing the weight listed on the tin. However: There is a metro article from last year which appears to be based on a Facebook post of someone who was not happy with their purchase: https://www.google.com/amp/s/metro.co.uk/2020/09/16/woman-calls-lindt-chocolate-tin-pathetic-and-advises-not-to-waste-your-money-13278693/amp/. The article mentions "LINDOR Christmas tins contains an average 32 individually wrapped truffles, double the amount contained within our bestselling 200g box of LINDOR truffles" This year's tins are already in the news: https://www.google.com/amp/s/www.portsmouth.co.uk/lifestyle/food-and-drink/tesco-is-selling-tubs-of-lindt-chocolate-and-shoppers-are-going-wild-for-it-3377395%3famp and the article mentions "now with 36 truffles". Assuming each truffle weights (200g / 16) 12.5g it would make sense if the current tins are (36 x 12.5g) 450g.
Probably net vs gross weight. Any photos of the bottom of the box?
Tesco website says 450g grams but Wilko and Ocado say 400g. Perhaps different sizes or Tesco include the weight of wrappers/tin or possibly just a error? Tesco seem to have the 450g one and the 400g listed on their website: 400g: tesco.com/groceries/en-GB/products/306661296 (listed as currently unavailable). 450g: tesco.com/groceries/en-GB/products/309490648 (currently listed as £10). 400g appears to have been the weight of the tin last year: https://www.squaremeal.co.uk/restaurants/news/lindt-sharing-tin-christmas_9797