If you use a bookie and convert all the odds on offer in a race to percentages (for example 4/1 = 25% chance), the toal percentage would be in the region of 115-117%. Obviously there is only 100%, the extra 15-17% in the bookies built in profit. Hence, over time, you can't beat the bookies unless you have a very good angle.
Look up 'overround' on Google for a better explanation.
Thanks,
So just to be clear, if I had bet, say, a grand on every horse in the race (39 runners by my count), therefore spending £39,000, what would my return have been?
Potentially stupid question (and after the horse has bolted, so to speak), but what is the downside to betting the same amount on every horse in the National (to finish in the top 4)?
I know it would have paid off in this race particularly, with the winner being 100-1, but as far as I can see whichever 4 horses came in first would result in you making your money back at least (just, if it was the lowest priced horses).
Am I missing something?
A grand to win? - You would have won £100,000
You would have done it differently. You'd have bet different amounts on each horse to return the same amount.

40 bets at £10 per bet = £800 (e/w bets are double the stake)Thanks,
So just to be clear, if I had bet, say, a grand on every horse in the race (39 runners by my count), therefore spending £39,000, what would my return have been?
I think that betting each way nets you half odds for a winner and quarter odds for 2nd to 4th (someone correct me if not).Thanks,
So just to be clear, if I had bet, say, a grand on every horse in the race (39 runners by my count), therefore spending £39,000, what would my return have been?
I think that betting each way nets you half odds for a winner and quarter odds for 2nd to 4th (someone correct me if not).

A man who put a total bet of £30 on three of the top horses in the Grand National has won £65,000.
Jason Darmody, from Swindon, selected three horses to finish in the top four of the race.
Bookmakers William Hill said the win, which had an 11,848-to-one chance of coming in, was the biggest the company was aware of for this year's National.