Another Dave wrote:

My wife has just been cleaning the gas cooker. It's seems to have been a
struggle and taken forever.

Oven Pride is a gloopy caustic soda solution, you pour half of it in a
strong poly bag, and put the shelves in it, for a few hours, agitating
and turning. You can wipe the other half on the oven floor if it needs
doing.

Obviously take care with the liquid ...