The answer was living on my desk in the Premises Licence Manual produced by retail, licensing and training consultants Lockett & Co -
a manual so weighty and upstanding that it forms a book end, propping up all the other stuff on my desk and one that I recommend to any licensed retailer (ring 01562 864488) as it is both comprehensive and user-friendly.
I quote: "If you wish to refit or change your premises, dependant on the amount of work you intend to carry out, you may have to seek a variation before you start work or, under certain circumstances, even make an all-new application."
I guess it's a reasonable assumption that if you had to submit plans showing where your booze is housed when you first applied for your licence, then the authorities will want to know about any changes. And do it before you start work.