Thanks Follower, Kuk.
I have considered more buttons in fact there will be six in total - the original http://en.wikipedia.org/wiki/Microwriterhad 6 buttons as one was for "mode" to cycle between caps, punctuation, numbers etc. But for now there are five as I want to keep things simple to start.
You should be able to check for multiple presses because the rule will be - "count which buttons enter the on state until a button is released" for example, the user wants to press 1,3 and 5 to represent the letter T - as soon as a key is pressed, the software should check for ON states and add them to a value, once any of the keys change to OFF the value is looked up in the table and passed to the serial port.
I think this should work - I got the code from Yerg2k excellent switchboard project which elegantly reads the keys with no false "bounces" - now I just need to modify it to do the looking up, but I am only a beginner coder!
Thanks again for your interest and advice - most helpful!
Regards, Ade.