It is a shame to hear that your theatre experience was ruined by such inconsiderate people. As a house manager and usher, I see behaviour like this frequently, and it is becoming more commonplace as shows struggle to appeal to a broader audience. In your situation, it is a shame to hear that ushers probably didn't handle the situation as well as they could have. They really should have been onto it the minute the group started moving. Any movement in the auditorium is distracting, and should be avoided at all costs. Because it was not dealt with by the ushers, and the house manager himself was not present, it seems he was left with no choice but to offer you some new seats, as he didn't witness the incident.
Hopefully the House Manager had a stern word with his ushers, and they will be more observant in the future.
"I will join this conversation on the proviso that we stop bitching about people. Wigs, dresses, bust sizes, penises, nightclubs and bloody Kylie!" - Bernadette, Priscilla Queen of the Desert the Musical