Ok the thing with Baumatic is they went bust last year so when a manufacture goes bust they stop making spare parts. Very common parts you can still get but other parts can be hard.
If British Gas cannot get the part then its very likely no one can. You find local companys have trouble getting parts but bigger ones can get them. So if they cannot get them its going to be likely you wont find that part sorry.
So if this is th case then yes you will be best getting a new oven from a brand that's still going and making spares.
If you need anymore info just get back to me.