Les,

One way to increase the take is to buy your own booze from a supermarket (or better still, a real ale wholesaler), and staff the bar with volunteers. Sell the drink cheaply, but above cost price, and donate the difference to charity. Reward bar staff by allowing them to purchase any unsold stock at a reasonable discount. Your local publicans will probably take a dim view of this - but provided it's a one-off charitable event and not a regular commercial operation, they have no serious grounds for complaint. Alternatively, if you need to stay on good terms with your local pub landlord, offer them the bar franchise (on reasonable terms).   

Another good way to increase your takings is to organize a raffle. Ask around for donations (unwanted presents, home-made cakes, etc) and supplement them with a few bought-in bottles. Calling the numbers and distributing the prizes will give the band a welcome break.

Wassail!