Hi griz o66,
As you can see from everyone's advice, most mainstream Trailer brands (eg, Dunbier, Mackay, Redco, Rule, Sea-link ect, ect) are fairly simillar in design. They do have slight differences however as I found when I went looking for a trailer. I have bought a Dunbier Centreline 4.2m trailer which I love. Compared to other brands, it was better reinforce (stronger gussets), better welding, made out of stronger box section and overall I thought it's finish was nicer than most.
I did look at Redco and Sea-link trailers as well (and to be fair they looked like pretty good trailers) but the Dunbier won me over. The size trailer you are after will be bigger than mine so I guess the features of the trailers will change.
I have had my trailer now for 8months and it still looks like brand new. There may be a lot of good trailers out there that can do the job but from my own experiance, I would give Dundier a go.