it works up to DEC255 then ouputs a zero for 256 etc, which is larger than a BYTE I'm thinking.
Yup, remember when counting not to start at 1 like we were taught in school.
Always start at the origin... ZERO

Hmm, a little stumped here. To my mind (after a lot of reading but obviously not enough!) when the first BYTE (number) arrives I need to send it to a memory location where it won't be overwritten when the second BYTE arrives.Then add the two together.

The only thing I've found so far that has a seperate address element is this (I think):
I was thinking along the lines of something like this for now
Code:
NUM1  VAR  BYTE
NUM2  VAR  BYTE
I was saving ARRAYs for a little latter
I'm thinking: MYVAR VAR BYTE [1] which is two elements 0,1. then somehow add element 0 to element1.
But if you want to read ahead...
http://www.picbasic.co.uk/forum/showthread.php?t=544