You can get ATF replacement kits fairly cheap from Germany via ebay
https://www.ebay.co.uk/itm/Original-...frcectupt=true
or via their online shop a bit cheaper afaik
https://www.ws-autoteile.com/en/. Thats all the same as the OEM BMW stuff - the BMW bottles are just rebranded. Transmission specialist charged £100+vat for the labour.