Does anyone know what thread and size the brake bleeder screws are on a Boatmate trailer?

They can be found here

but it doesnt state the size of thread, etc.

I am located in the UK and need a the postage from USA to UK will overwhelm the cost of the parts.

Even a cross-reference part # might help?

I assume they are Imperial thread.....

Have to replace cause they are stuck in badly, and looks like i'm gonna have to drill them out.