Because I didn't want the temptation to use it. Especially if I found myself in a challenging position. I'd far rather have it brought down to a limit I would choose. They increase it precisely to tempt you to use it, after all.

At one point I had as much available credit across two cards as balance on the mortgage, whereupon I decided it was silly and I'd like them to reduce please.

OK thanks. I was worried I was negatively impacting my credit rating somehow. My few minutes of research indicate that having a higher limit may actually be beneficial to someone like me who never utilizes my available credit.

The ratio of available to utilized credit definitely does impact your credit score positively. That's why for a while, I would periodically ask for limit increases 1-2 times per year on all of my cards even though I will never come close to the limits. You don't want to do this too often though; I think asking for an increase may incur a credit pull. Too many of those can hurt your score.