Values do not parse through to a custom new item form


Hi, when trying to parse values through to the custom new item form of another list the values are not being put into the list form fields, I've tested it and they parse ok into fields on a normal (standard) form but not into the custom one. Is there any code I should add to the custom form to get the data in the query string to reflect fields?
One note; on this page I have hidden the standard new form (as per usual when customising) and when i unhide it, to check if it is receiving the values the standard form is doing so.