Unable to select next page from ajax site

Hi

I am trying to scrape this site http://www.sjhallplant.com/stock-list and have used a proxy session to obtain the post data (needs a key and page number) but when I run it on screenscraper I get a status of either 400 or 404 depending on the variants that I try when I include the page parameter.

Alternatively I just get the first page of data - selecting the pattern for the 'next page' is easy - applying it is the tricky bit...

I am not sure if it is token or cookie related - I have a feeling it is somewhere in this area?

Any help would be gratefully appreciated.

Thanks

Jason