If the cooker has a flying lead from a cooker connection unit(a connector fitted between the isolator & the cooker) to the cooker then you could just break into it & fit this type of isolator http://www.screwfix.com/p/mk-metal-clad-20a-double-pole-key-switch/29973 in the cupboard as you have said you would like to do as long as the cooker was not rated higher than the switch allows
I'm not sure of the cooker rating, but it is an older style freestanding cooker on a 32A breaker, I'll check the rating,
This is similar to my suggestion in my opening post although I was going to use the rotary isolator switch which was the only lockable switch I could find with a higher rating. The fixed wiring already in place would not be touched just break into the "flying lead"
Thanks for your reply