First off, congrats on getting the gig, wish Cornwall was a little nearer and I'd be there.
Secondly, you are a proper musician, in a proper band, doing a proper gig (whatever proper means), so hold your head high, don't worry about the other band and do what you do well. You are there on merit I assume?!
Thirdly, I have never met the other band so I can't make an informed judgement, but be prepared to be welcomed by them, or ignored. Headline acts can be great towards the other bands, or complete twats, just like life. People are people and sometimes there behaviour (good or bad) can be exaggerated when they have a guitar/mic in their hands.
Got drunk with Southside Johnny & been ignored by headliners in pub bands........nuff said.
You just be you and don't forget to buy the sound guy a pint (but not too many before your set!!), it's worth it.
Good luck with the gig, enjoy.