I found something from the Welsh tourist board for B&B proprietors. Click on the PDF, A Practical Guide to Legislation. http://www.wtbonline.gov.uk/server.php?show=ConWebDoc.634

Notice that the chart is identical to the one your B&B was using. However the tourist board chart is clear that it is the percentage of the full stay, not just of the deposit, say.

Condolences on your loss.

Frances Toronto, Canada