What do you mean about ticking the box for every individual manufacturer?
You can just tick "Pre-Owned", "Wagons" and "OO" to bring up all pre-owned OO wagons, and the numbers next to the tick boxes tell you how many results your filtered search will come back with. Having established a broad category this way, you can then search within it if you're after something specific.
It seems very intuitive to me, and much more useful than the old website. The ability to filter by multiple fields across multiple categories, and indeed to search those categories for specific fields - in effect searching for search terms, is a godsend.
Unless of course I've completely misunderstood your post, in which case I apologise.