I have been looking at this more - and now have a number of things to think about. A lot seems to be talked about getting the bet size correct in proportion to the amount that you have in the ring fenced area to bet with.
Some people seem to use (as you say) a flat amount (2% of the starting total seems to be popular) to bet with for each bet. Others seem to use a strategy such as optimal betting (I liked the explanation here The Mathematics of Options Trading - Google Book Search which looks at how to calculate 'optimal f' and also shows that if you get the amount wrong you can actually loose money).
We do however also know that there is a guaranteed % return on the bet and have a good view on the probability of the outcome occurring.
Looks to me that putting these things together in some way should get a better outcome than either just fully laying or flat bettering would? The solution will also need to factor in that there are limits and its unlikely that for a lot of bets you would actually be able to place the amount calculated.