First.. Great looking Bulldog!
Second.. Rugby! Love the game! My oldest boy plays for one of top ranked High School teams here in the states. He has to sit this year out and its killing him! (and me.. i'm going to miss watching the games).
Third.. Are you sure its only bags? Not piles of clothing.. Dog Beds... Blankets... etc? (have you gone around and sniffed everything?) If he's marking his territory he is going to pee on ANYTHING and EVERYTHING! (I had a dog that would pee on people to mark them

) Where does he normally go pee? I ask because some dogs get into the habit of going potty on certain types of surfaces. I don't know how a plastic bag could remind him of his normal potty 'zone' but you never know.. maybe color or sound?
Or.. he could just be doing it for the heck of it.
Either way, i would set him up to fail. Put a few bags down and wait.. as soon as he goes to do his 'business' stop him and take him outside. Then repeat, repeat, repeat... etc.