Caol ash contains sulphur and when it gets wet produces sulphuric acid. So, I am afraid that a mixture of coal and wood ash should be put in the bin.

I have had to consign so much to the bin :o(

I'd strongly recommend that you don't use coal ash on the garden. The reason that so many old gas works are contaminated with dioxins and heavy metals is because of the all the coal they used to make gas.
Coal ash contains a horrible mix of toxic chemicals, many of which are absorbed by plants. I wouldn't grow vegetables anywhere near it!
There are lots of alternatives which won't put your health at risk.