I hate to be the bearer of bad news but you're exposed to potentially serious trouble here and will need robust legal advice, urgently, with a view to possible legal action against your tenant.
The bad news is that a previous finding from the Ombudsman, detailed in their 1998 Annual Report, concludes that it's unreasonable to expect a residential tenant to assume responsibility for their landlord's tax compliance obligations.
IANAL but if your tenant cites this either in seeking to breach the tenancy agreement or in court, the implications are obvious.
Hopefully it will not come to that.
In the longer term, you need to drop altogether the idea that your tenant deducts tax at source.
It's a recipe for disaster and a monument to the bovine state of tax legislation in this country.