What oven is it?
How did you remove the old oven?
There should be a switchable socket like this
https://www.wickes.co.uk/Wickes-45-Amp-Cooker-Control-with-Neon-Slimline---White/p/711331
The 13 amp socket is NOT for the oven/cooker. You need to hard wire the cooker straight in to the back of the...