Removing water valve on Main Multipoint BF

[Toddmin]

Customer has a Main Multipoint BF that needs the diaphragm replacing. I'm attempting to remove the water valve but one of the screws that fix it to the bottom of the gas valve just won't budge. The head of the screw is completely stripped. My trusty neji-saurus screw pliers turned it about 45 degrees but won't go any further.
Any suggestions short of drilling it out?

​

 

[PETE 123]

Took me an hour once to remove one wd40 heat and tapping screw helped. stainless screw in corroded ali bad design.

 

[Toddmin]

That's pretty much what I was doing for 2 hours on Friday but no joy. Kept thinking that it would surely loosen up eventually but never did. Said I'd be back on Monday but feel slightly embarrassed just doing the same things again when I don't seem to be getting anywhere.