Hope you don't mind me keeping this thread going / hijacking it.
I've just been out & had a look at my cooker & taken a few pics.
The control valve does look quite simple - from the outside. It still looks like a simple spindle which should be easy to copy on a lathe. I have one but can't really offer to help as I have so many things on the go, but someone else may be able to help.
If you can manage without the grill it looks like you could unscrew the valve from the manifold & plug this with a bung. It looks about 1/8" poss BSP or G thread.
I talked to Kev - Ex Delilahtoo, about his Dudly cooker & I'm sure he has his grill disabled.
I'm sure you've seen this but here's a pic - taken with dirty mirror.
Here's the hijack bit..... When I use my grill the whole cooker get very hot, including the inlet gas manifold. It looks like most of the heat from the grill rises up & get trapped under the cooker. There is a slot in the centre above the grill but there's no mechanical means to help the hot air go in that direction. Here is a pic showing the underside of my cooker. Is ther a missing heat deflector or anything else?
I'm thinking of fabricating something but I could copy an existing missing part if it exists.
Phil.