Every year, for each car in the family fleet (6, excluding the modified 20vt), I go the 4 comparison sites (Meerkat, Annoying-tw@t-with-the-'tash, Supermarket, Confuzzled) and update accident/points history if required (!) and run through different mileages/excess options and see if something jumps out. Sometimes having a lower voluntary excess makes little difference to the premium.
It used to be that all the comparison sites pretty much came up with the same thing, but I've noticed that sometimes one of them will offer a lower premium for seemingly the same insurer and same cover, perhaps that's to do with referral payments or something, but see footnote (*).
Then click through the best ones to see if details have transferred correctly (*), also try final provider's own web site, and try others like DirectLine etc. for a laugh (sometimes hopelessly uncompetitive).
This approach is OK for non-modified cars, but in my experience although you might save in the first year when you set up a multicar policy, come renewal time the quote is just as rubbish as for individual policies renewing. I've yet to be convinced that a multicar policy make sense over time.
It depends how much time/effort you want to put in vs. the potential savings. Even trying just one comparison site will give you an indication of how (un)competitive your renewal quote is.
(*) details sometimes get mangled, there was an article the other day on BBC News