At a guess, the convection heat element has failed (probably th eone in the bottom of teh oven).
Check out this page for spares:
http://www.espares.co.uk/search/mo1557902pt1595/cookers-and-hobs/elements/kenwood/ck404
This may help:
https://www.youtube.com/watch?v=iOZZo8P1z9M
Other causes could be the control switch or the thermostat.
check connectors are intact and seated securely.
Check wiring is sound
If you are confident working with things electrical it is easy enough to check power is getting to the element - just use a test meter or even a 240V bulb and some temporary leads - BUT ONLY IF YOU KNOW WHAT YOU ARE DOING.
You can also discomneect the element and check the continuity through it with a test meter or battery and bulb.
Look at the element: if there's splitting along the sides, replacement is - or will shortly - be needed.
X, February 2015