I have cut and routed most of the wood required, but made the faux-pas of ordering 12mm guide rails and routing 1/2" slots in the guide rail supports!

For the sake of another £10, I have generated a cutting sheet from a stock 18mm MDF 1200x600 sheet and will be going to the DIY tomorrow to buy a sheet and get the whole lot cut down to workable sizes (I estimate 11 cuts).

I will route the necessary pieces using a 12mm square router bit, and then separate the smaller parts using my bandsaw.