Thanks for all the replies.
After shopping around and no luck,I walked into a hardare shop for something else and as I was walking out ,there it was ,a 5.5mm router bit.
For anybody in Australia thats having trouble finding these, they are made by Carbatec -T Cut and cost about $25.00 AU.
I was told by a lot of expert tool shops that they were not available.
I have cut the slot and its a nice neat fit.
Thanks
Greg