thanks for the suggestions. I think I'd checked at least one of those. Problem seems to be that you'll get a 40mm wide bolt up to about 115mm long easily enough, but the trailer's fitted with 40 x 155s.
In the context of all that I'm doing to the boat, 4 bolts at 7 quid or so isn't a big deal, it's just the principle behind it, and the feeling that someone, somewhere must be doing these cheaper and as soon as it's for 'marine' use or a 'boat trailer' the price climbs.
As I say though, thanks for the suggestions. Maybe I should email them and see if they have non-standard stuff that's not on their website.