Unless this english mastiff I saw was a young one, it seems that they are also a little smaller body, but a beautiful face. I have always thought of a bull mastiff as being a very large dog, but maybe this was a very large pup..
http://www.dogbreedinfo.com/mastiff.htm
http://www.dogbreedinfo.com/bullmastiff.htm
English Mastiffs are much larger than Bull Mastiffs. IIRC, English Mastiffs take about 4 years to reach full growth, so the one you saw may well have been young.