I finally started my rails project on the weekend. I've used an inner rail of 50x50x4 SHS and an outer rail of 32NB medium wall pipe. I was originally going to bend the pipe but found my local steel supplier had the corners that I could just weld into position - and they will be much neater once it is all finished.
Here are some pics of the process.
Using these pre-fabricated bends meant I just had to focus on a good solid weld (not an issue!). The smaller pipe shown against the square inner rail has been cut at a 30 degree angle so the outer round rail will sit up (see later pics). As a result the outer rail is about 1370mm - 1380mm long.