How can I get my lower oven to heat up?

Stoves, Newhome Gas Cooker - SI550dom.
I switch on my lower oven and the flame ignites and turns on - no problem. It just doesn't heat up enough to be able to cook anything.

* it heats up as and when it suits
* I turn it to gas mark 9 and the flames do not ignite fully so no heat available to cook.
* my hubbie has looked at thermostat and given it a clean - but still no heat.
Please can anyone advise my next step.