Are the pads newly fitted? If yes, genuine Vauxhall (preferred) or aftermarket? If aftermarket, may be too hard and are, effectively, racing pads that require heat before reaching optimum operating temp. When they cool, back to wooly brakes again.
Car new to you? If yes, then see last couple of statements above re hard pads. If no, then thinking more a stuck caliper(s) causing pad(s) not to release clean from disc, gently rubbing and causing reglaze.
Park on a very gently slope - empty car park type camber - and release the brake. Car should gently roll downhill. If not, brakes are binding a tad/calipers not releasing.
Once it all cools down again, (tomorrow morning best) then go for a gentle drive (couple of miles) trying not to brake for as far as possible or if you have to, brake lighttly. What is then suggested is to (gently, it may be hot) touch each disc around the car to see if its hot. If yes, then problem wheel found, see sticking caliper option. If no, then kinda stumped!!

HtH
B