Is it one that opens with the door rather swing away? I bought one from Paddocks a couple of years ago, don't know if its Britpart or not but it has been fine and seems reasonable quality. I found the trick was to fix the lower mounting first as that gets you in the right place for everything else, that goes onto one of the bolts along the top of the rear cross member. Have a look through my chassis change post, you will come across a few photos that will show where the brackets go.