Replacing Pressure relief valve on Baxi Boiler

I am replacing a leaking pressure relief valve on a Baxi Bahama 100 Combi boiler. 2 questions:-
1 The grub screw holding it in is seized and the slot is sheared. Any suggesstions on how to remove, apart from drilling out.
2 Once I get it out, what is the spec of the screw.
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