Same thing happened a few years ago with one of our bullmastiffs.

It started with him acting strangely, trying to stretch and unable to settle. Then Anita noticed his stomach looked like it was swollen. She put him in the car and went off to the vets. As luck would have it there was a blizzard that evening and the motorway was having the worst 'white out' seen round here in years. By the time they got to the vets it was too late and he had to be put to sleep. This was all in the space of about 2 hours.

You were very fortunate Adrian, prompt action definitely the key here.