Considering using these "double" pipe clips up in the loft:

 

 

http://www.screwfix.com/p/talon-15mm-double-hinge-clip-white-50-pack/20167

 

The runs will be in what is a potentially cold roof space ( in the nom 1' gap above the ground floor ceiling joists and below the dormer floor joists), at least in the short term. I'd want to anyway insulate all of my pipes. If I were to use these double clips to run a hot and cold 15mm side by side, then insulated over the whole lot, would I not risk masses of condensation between the two?

They leave a gap of about 20mm between the pipes. 

 

What we used to do was to slice the top off both pieces of insulation along the slit side to make a "D" shaped section. These then sit tight up to each other and just get taped at the joints top and bottom. 

 

You end up with an insulated figure 8 that works fine. 

+1

but it's a lot easier to use them with these spacers too. Gives you a gap to get around the back of the pipe without fouling on the surface the clips are mounted to.