Come right out with it. Tell them what you've decided in the first or second sentence. If they're due an apology, do it there.

Most people don't. Instead...

...they go through all the background, telling you what you told them, the date of your complaint, the name of the product. We just want to know whether they accept the complaint and what they're going to do. We scan the fluff for it.

How about something like this instead?

Let's also talk to Mr Jones as if we were in the same room, not address him as if we're a 19th century bureaucrat.

If we acknowledge the frustration or the inconvenience, he will know we've really heard him - not just the facts, but how he felt about the experience.

'Thanks for complaining'

Yes, thank your customers for letting you know they were disappointed/frustrated/inconvenienced. It's better that they give us a chance to put things right than tell half a dozen people how upset they are with us.

Most people are very forgiving if we are straight with them. A supplier of ours with an impressive record for service, used to ask anyone who complained how they would like to see their complaint resolved. Almost every complainant suggested a much more modest solution than our supplier was happy to provide.