Hello Simon,
I have to assume that you do not have gas fitting or servicing experience, otherwise you would not have asked the question on this forum.
There are a number of things that could have happened to cause you difficulty, but unfortunately virtually all of them would require the remove of the oven or some part of the gas system to establish the fault, and to effect a repair.
For your safety I have to recommend that you seek the professional services of a Gas fitter to attend the appliance.
They will be able to quickly establish the cause of the problem, and Carry out the necessary repairs in accordance with the Gas Safety Regulations and all relevant Health and Safety protocols.
If you attempt a DIY repair, your insurance may invalid, and of course you may endanger yourself and your family.