The cost of making good the property and the cost of bailiffs/eviction will be tax deductible (as they are not of a capital nature).
With regards, to the letter, I expect that if you were legally obliged to provide answers it would be clear from the letter under which legal provision you were obliged to do so. If HMRC wrote to you simply because you were on their records as the client's agent, I suppose you could write to them to explain that you have not been engaged by your client to respond to questions relating to the tax check?
Thanks for reply..I forgot to mention that my client then sold the property, rather than taking on another tenant. Does that make any difference? I will have a look for a phone number as you mentioned but i may just write to hmrc to tell them my client declined any help etc.