When the writer says "It was no good. He'd have to...", she is telling us what is going on in Mr Dursley's head. In direct speech, it would read like this: Mr Dursley said to himself, "It's no good. I'll have to say something to her." (present and future) When the writer reports these thoughts to us, the grammar transformation is the same as for reported speech. Present tense becomes past ( It's --> It was) and 'will' becomes 'would' (I'll have to ---> He'd have to).

Su.Ki gave an excellent answer. Here's more information: https://www.ef.com/english-resources/english-grammar/tense-changes-when-using-reported-speech/ In the poster's example, "would" is not a conditional. This is a form of reported speech so "I will" has become "he would." Example of reported speech: I met Joe Friday. He said he had changed jobs and that he would travel a lot after his training period. (Original statement by Joe: "I have changed jobs and I will travel a lot after my training period.")

He’d have to = He would have to. It’s a common construction - pronoun + to have + would (not necessarily in that order), meaning that the person or thing concerned has to do something. It’s a conditional clause, in this case the character speaking needs to ‘say something to Petunia’ if he wants to clear up the question about her sister, whatever that is. If it were ‘he had had to’ or ‘he’d had to’ then it would be in the past perfect tense, i.e referring to something that had already been resolved, ‘he had had to speak with Petunia about her sister in order to....(whatever)’. ‘He had have to’ doesn’t exist in English as it mixes past perfect and present tenses in a way that doesn’t work.

Jonny, "He had have to say something.." sounds right to you??

Both seem right to me