Shower 8.5 kw need to know if I need to put a longer cable

I think you mean 10mm and not 10m. If the existing cable run is compliant and accessible then it can be reused. It will need to go all the way to the consumer unit. The shower needs an isolator as well. Remember that this work is in a special location and is notifiable to Building Control. You will also need an Electrical Installation Certificate.

Pull the old cabke out. Make bigger holes and run the new cabke from the fuse box to the shower, ensuring it has correct fuse rating along with rcd protection. Best advise is to always get a qualified and trained electrician as they will definitely know how to put a cable in.

Limited information, do you have RCD protection? Is it recently fitted? Where is the cable running and where is it burnt? If the cord burnt out then could be loose connections, wrong fuse size etc. The RCD or fuse should have gone before the cable if sizing and calculations all good etc as that is the point of the protection. Call an electrician and refresh the whole system by using a professional. Don’t mess with it yourself as it’s special location and need certification.

It’s not a longer cable that’s needed it’s a bigger cable 10mm and the MCB at the fuse board will need to be up rated to a 40amp one

Yes you can, 10mm in one lenght, but it has to be seperate circuit and suitable auto fuse rsd, and free of moisture (properly seals within shower unit