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The chalky 'natural' does look as though it will be free draining. As to 'poor' I am not sure how much that will matter, many alpines grow in places with soil that makes that look rich! However the climate in the mountains will be rather different to that on the coast of S England, so you may want to add something to help with moisture retention. Soils over limestone are generally species rich as the lime helps to make some nutrients available in larger quantities. However advice from people who garden with similar soil will help you (what do you think Gerry W?)