Essentially the strategy would consist of:

(1) Place bets on all individual Tory candidates (with varying stakes but equal potential payouts), to give a gain that increases linearly with the number of Tory seats, ranging from a loss if they do badly to a gain if they do well.

(2) Supplement this with a number of smallish, well-chosen bets on some of the lower bands of the total number of Tory seats.

Playing with current odds, I can get a graph of gain versus Tory seats that is positive *almost* everywhere, and the few worst-case places where it is slightly negative (which are if they just scrape the bottom end of each of these bands) the loss is relatively very small. The total amount bet on these bands works out a lot less than the total amount bet on individual candidates. With 50:1 offered on the Tories getting 200 seats or fewer, it is cheap to safeguard against what would otherwise be a big loss in this event, and then some bets on the next few bands are needed to deal with some other losses for moderately low Tory totals.

I am not advising anyone to put this into practice, but the principle is there. There are no doubt also practical variants where one backs Tories in a number of selected marginals rather than laboriously all their 600+ candidates, but opening up an element of risk if the party does well overall but not in the selected marginals.

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