what size cable is ok for an old belling electric range cooker?

I'm sorry but that is not right, 15kW is nearly 63amps. Usually like you said it's normally protected by a 30 or 32 Amp circuit breaker but it all depends on the demand of the actual appliance.

The rating of the circuit is determined by the assessment of the current demand of the cooking appliance(s), and cooker control unit socket outlet if any, in accordance with Table A1 of appendix A. A 30 or 32A circuit is usually appropriate for household or similar cookers of rating up to 15KW. ADDITIONAL NOTE TO RS INSTALATIONS: The above statement has been copied word for word from the on site guide BS 7671:2008(2011) (green copy) appendix H section H4 on page 176. These are theIET wiring regulations 17th edition all part P registered electricians are required to work to and I know it was exactly the same in the previous edition. Here are the calculations as stated by the onsite guide. Appendix A table A1 Household cooking appliance The first 10A of the rated current plus 30% of the remainder of the rated current plus 5A if a socket outlet is incorporated in the control unit. i.e. 15kw=65.217391A less 10A=55.217391x30%=16.565217 10A+16.565217A+5A=31.565217A So according to the official calculations the original statement above is correct. I know a lot of guys do not know this which is why I listed table A1 appendix A then carried on to do the calculations as laid out in the same guide. So feel free to check my statements in the on site guide - all electricians are required to have one.

You need to get the Kw rating of the oven e.g 9kw and convert this to watts (9000) and then divided this by the voltage of 230v this will give you the current drawn under full load. Once you know this its then possible to assess if 6mm cable is adequate or not, but with two appliances connected to it, 10mm cable would be the best option.

So much rubbish is talked about cooker circuits with well-meaning electricians treating them as if they are showers. With cooker circuits, you have to apply diversity as Kevin Cassidy has the other two answers here are not right, a 6mm cable can generally be sufficient for up to15 kW of cooking facilities as found at H4 page 212 of the current onsite guide.