Thanks.
Do you (or anyone else) have any experience of screw extractors?
I'm guessing it's probably worth a try before I destroy the threads by drilling!!!
Are you sure its stuck not just turning around easily?
Asking because i thought mine was stuck and rounded but in fact when i took of the pipes in the engine bay and wriggled it all fell apart.
Sometime a flat head screw driver can get a grip.
Make sure you disconnect the hoses in bay and its 10000 times easier.
I didn't cut anything or dismantled the car as some pictures show and can be done with a long screw driver from hevac or radio unit when out.
Got some 500 mm long screw drivers and its easy, sure you cant see but take a photo from foot well up towards the thing and it can be done by feeling,
get a helper and its a 1/2 hour job, most spent on draining/refilling coolant.
Careful not to over-tighten the new screw and you have bigger problems, i just fitted a new 30 mm multi head and it doesnt need tighten much, its aluminum you're dealing with, also found the length doesn't matter too much i.e. slightly to long no problem- just more turns, nothing behind it.