Ah yes, I probably wasn’t clear enough at the bottom of my post: it’ll be a gas safe / cooker specialist who I get to repair. Definitely not a DIY thing!
Yes, I can hear the sparking when I try and ignite the two that won’t light, I can’t see a spark from either of the ignition points.
The thermocouple is the bit I was looking to know the name of, thank you! It’s definitely not dirty (well, not as far as I can see), I have looked after this cooker and it is cleaned regularly, all the obvious bits get taken off although I draw the line at unscrewing things (I am safety conscious!)