Cannot answer your question, misses, but the Surrey and Middlesex Stations have seen a very similar number of wins. Since given the choice (certainly in recent years) crews tend to choose the Surrey Station ( to take advantage of the long Surrey Bend about a mile after the start) and given that the toss has probably been won about 50% of the time by each team it would suggest that the side losing the toss must have won the race on quite a number of occasions.
Sorry, mattk, but you really should grow up a bit. Not all lads that go to Oxbridge are posh and not all lads that row are posh. (I rowed for my school and I am certainly not posh, not that there is anything wrong in being so). You should also realise that there are no sporting scholarships available at either Oxford or Cambridge and rowers are selected on their rowing ability not their academic prowess (or their poshness). Having said that there is certainly enough evidence to support the accusation that both universities have parachuted in world class rowers who may otherwise not have been at the university. But they are still not necessarily "posh".