Welcome Paul,
Would this screw/bolt have an approx 10mm head?
There are ‘special socket’ tools designed to remove mangled head on bolts and on nuts - try Screwfix and/or Halfords, or your local motor factors.
Can’t get it to turn with a Mole grips or similar?
If all else fails, you will need to carefully drill through the head remains so it falls off.
I would go straight to your dealer for a replacement fixing, rather than shopping around to try to find what you think you will need. Obviously be more expensive at dealer, but….
Kind regards,
Gareth.