I think i've got it?
Thanks for getting straight back to me but I think i've just worked out how to do it, if you select the 'ecart add from recordset' behaviour on the product results page or page containing add to cart buttons, it lists all the columns with an option to make them hidden so you can add an extra column in the ecart object which then appears in the server behaviour 'ecart add from' and from here you can make it hidden. I'll have a go of this and will let you know how i get on. Fingers crossed. Hopefully i can make it work but if it doesn't, i'll definitely re-attempt to get the join done but first i'm going to alter my ecart setup slightly and see what happens.