Thanks for the input guys, my current piece that I use for the sub base has paid for itself in accuracy for depth of cut already. If was only £4.25 with free shipping!
The 10mm thick piece I believe comes in an a4 sized sheet which can be trimmed as needed.
I also marked 2 clear cuts with my square along the edges of my bit which enables very accurate measuring. It was very accurate when routing out dados for the bench dog assembly, which is a part of my current bench building project. Despite lots of cuts and use the measuring lines are still very clear
When the bench is completed, which should be in a couple of weeks (free time at a premium at the moment!) I'll have a good look at some simple router table designs and make a start.