As you have only got a 1/4" router, to avoid putting undue strain on the motor I would go for a 1/4 or 6mm x 25mm bit. Normal TCT should be OK as you are obviously not after the best possible finish and a little tear out can be accepted.
As Jason says because you are only using a 1/4" router avoid putting undue strain on the motor by using a 1/4 or 6mm bit. Again as you are using a small router and the wood is 20mm thick do not try to cut through it in 1 pass, take at least 2 passes.
If you want to stop any tear out put a scrap piece of wood underneath the good one.
It may look like i am trying to tell you how to suck eggs here but you did ask for any tips. Sorry.
Quite right too and one of my many New Year resolutions is to learn how to suck eggs
Thanks all. Reading through I think I'll use my 1/2" router, I thought that the quarter inch would be OK but obviously I'll possibly be pushing my luck and it'll give me a chance to try it out :wink: I've got a solid tungsten 6mm twin flute straight bit, could I use this ?