Someone has definitely complained mate. The Planning Enforcement service of your local authority is not set up to be proactive, so relies upon information from members of the public.
You should just write back to the enforcement officer and say that the use of your property is residential, that you do not conduct any business activities from the property and that your hobby is working on your own car from time to time which is incidental to the use and enjoyment of the dwellinghouse. Accordingly, there is no breach of planning control. However, if he has any evidence to the contrary, you look forward to disclosure of the same.
I would not bother providing a diary/log to him. The only reason he wants you to do that is so he can attempt to prove that the car repair/maintenance activities have gone beyond what can reasonably be considered a hobby. In other words, he wants to use your diary as evidence against you. Let him gather his own bloody evidence!
You do not have to give any other information unless you are served with a PCN (planning contravention notice).
Neil