You can put it on the garden in winter, Liz, and the rain will wash the goodness down into the soil ready for spring planting.
Have you thought of storing your leaves to make leaf mould. It is more beneficial for the garden than potash and dfoes not cause smoke pollution.

There is a blog on Spritzhenry's page which shows you how you can make leaf mould as Bulbaholic states.

Agree with the above - a waste of good resources to burn leaves! Much better for your soil if you compost them, as Spritzhenry describes in her blog A Recipe for Black Gold. I must say I'm surprised you're allowed to have a bonfire - most areas of Britain are now heavily restricted because of the air pollution it causes.

We are allowed to light fires here too, each council has its own laws and regulations, perhaps this is why. The biggest emmiter effecting the earths global warming are volcanos lol

Thanks. We are allowed small fires as long as they don't cause a nuisance, but I will compost half and burn half in future to avoid contaminating the air! Liz