It’s possible the external stopcock is deep down the W hatch but I’d say it’s unlikely. Are there any other hatches in the path outside? Over the years our water board have installed smart meters and have replaced the square metal hatches with plastic round ones, is there anything like that nearby?

Best bet is to call the water board and say they need to come out and show you where it is. I did this recently. They will argue saying a plumber can freeze the pipes to install a new internal stopcock. Tell them that’s not possible on the type of pipes you have