Hob switch as part of multi-gang?

The quick answer is no but It depends on the power rating of the hob and whether its supplied from the same circuit as the other appliances in the grid switch. Generally they are higher rated than other appliances and would be on a separate MCB. This isn't something should be attempting as a DIY job. This is something only a competent electrician should be doing

Depends on the rating of the hob but I would like to see the hob on a separate circuit.

You should put a separate switch for the hob , not on the grid switch, usually the grid switch have 20 A the current rating, and a hob switch have 40-50 A the current rating, and has to be on a separate circuit

To add a switch for a hob through a multi gang sw it would need to control a contractor to isolate the hob or a seperate sw would be needed it would also need to be on its own circuit some induction jobs are only 13a rated but not many

No