Hello Chris, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
In my opinion the landscaping of a garden is not part of a dwelling house with the definitions contained in VAT Notice 708 and cannot be zero rated; it is a standard rated supply. Furthermore I cannot see how any VAT paid on such a service can ever be reclaimed in the circumstances set out in your scenario. You can find the Notice here:
I am so sorry to have to rain on your parade.