There's a fine diagram on page 17 of the part P document (Electrical Safety - Building Regs) here...
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADP_2006.pdf
It shows an Oven and separate Hob powered from a dedicated breaker with a Double Pole isolation switch in the Kitchen.
This I guess is how a new installation should be made to properly comply with the regulations.
If it were my cooker I would not lose any sleep over it - your new electrician may just be right in what he says and in these quiet times he might also be looking for a bit of extra work. Any illegality is doubtful and would be the responsibility of the original installer - not the householder.
A keen advocate of the regulations we now have was Baroness Tonge (Dr Jenny Tonge Liberal Dem MP for Richmond) following the tragic death of her daughter...
http://www.telegraph.co.uk/news/uknews/1473919/MPs-daughter-electrocuted-in-botched-fitted-kitchen.html
Peccavi
May 2009