Best solution would be to just get rid of the 2nd car, as it sounds like you could easily manage without it. If you really have to keep it, you are just going to have to keep trying different companies. Bear in mind, plenty of them won't be on all of the price comparison websites, some won't be on any.

If you only need a 2nd car a few times a year, it may well be cheaper (than spending £500) to hire a basic car as and when.