Hi Tony,
Two of my friends have built run off rooof observatories and they are happy with the outcomes. Here are some details of the rollers and rails they used.
http://www.rollingcenter.co.uk/
The rail for these wheels are the “Angula Z3”. They come in 3 metre length
The wheels were £4.44 each – I got 6
the rails cost £22.64 for a 3metre lengtH
The ‘V’ groove wheels & rails are superb – it really is possible to push it with one finger.
Denis Buczynski