Hi Rosie. It's worth investing in the best trousers you can afford, because there's nothing more miserable than leaky trousers and sitting on a damp bottom all day! How active a sailor are you? Are you mainly sitting still in the cockpit, or are you running about on the foredeck? The more active you are (eg racing crew) the more wear your kit will get, and the more important it is to get stuff that is up to the job.
All the main brands do a range of different weights, roughly classified as coastal, offshore or ocean. You don't need ocean weight unless you are planning to do a lot of winter sailing, in which case the extra protection (and cost) is justified.
I would always go for offshore rather than coastal weight, because they are harder wearing and also tend to have have more pockets, better hoods, etc.
Things to look out for: reinforcement on seat and knees, adjustable fit, useful pockets (eg handwarmer pockets on the chest of hi fit trousers), breathable fabric. I like really hi fit trousers that cover chest and back: it means you can often do without a jacket on a warm day, and they keep you nice and warm when it is cold and wet.
Some makes offer drop seats to make it easier for ladies to visit the heads. But you often have to strip anyway to remove all the other layers underneath, and unisex ones are just as practical.
Men's trousers often have more pockets, too.
You won't go far wrong with any of the main makes (Musto, Henri Lloyd, Gill)and lots of people swear by "bargain" brands like XM.
But whichever you choose, buy the weight that suits the sailing you are doing (or more importantly, intending to do in future).
And be aware that yacht waterproofs are not ideal for dinghy sailing, where capsize protection is needed.
