If the landlord ordered the work to be carried out it is probably his responsibility to pay but it depends what the repair was for it may just be a mix up and each side (Land lord & Tenant) think the other should be paying. The tenancy agreement may set out clauses which stipulate who is responsible for what repairs, the landlord’s liability will depend on the precise wording of the relevant clauses. It is usual but not fixed for the landlord to be responsible for all repairs to the outside and fixed parts of the property, and the tenant most other areas. Be sure it is a repair not an improvement a landlord does not usually have to improve a property just maintain it.
There are some grey areas such as dampness.
Remember you may lose a customer by taking action, this is the unfair part, but why should work be done for nothing ask landlord & tenant to sort out who should pay, once you have established who is responsible to pay write and ask them to pay within say 10 days, if they do not write and head your next letter “letter before legal action” and give them one month to pay. If they do not pay, and you are not put off by the small amount start your claim in your local County Court where if they lose they will also have to pay the court fees.
If you do not want to lose a customer you will have to feel ripped off, not a nice feeling.