Spent mushroom compost has chalk chippings in it and is a good all round compost for putting back growing properties in your garden. You will need a large amount, a 6inch cover may do the trick. Take a PH reading after a week or so, if you have had a steady rainfall and it is still high add carbonate of lime, 4 ounces to a square yard.

This isn't exactly helpful but you could look at it positively and use the opportunity to grow things that love acidic soil! We have alkaline soil and it seems I love everything that won't grow in it - perhaps it's simply that the grass is greener...
Sorry if I've been unhelpful.

Part of the problem under a former leylandii is nutrient depletion. I would also add bone meal, blood meal, kelp meal, and maybe dolomite. The last would also help raise the pH.