This sounds as there there is a faulty control board on the PCB, which powers the timer and electrode to start the oven.
Unfortunately this part is rather expensive, and can be found here for £110.
With this being a gas appliance you will need a Gas Safe engineer attend, I would suggest this may be getting to the point of an uneconomical repair, if you would still like to proceed, and for me to recommend a suitable engineer in your area please provide me with your postcode.
Hi Rose, please let me know if I can be of any further assistance.
Hi Rose, in order for me to paid for the assistance provided to date please could I ask you login, and rate the service received using the star rating on the page.
Good morning Rose,
Appreciate if you can rate the service received using the star rating shown,so I can be paid by JA for answering your question,
Please let me know if I can be of any further assistance.