If you take a look at the 3rd entry on this page click here, it says;
"Compatible with Epson Stylus Photo RX420, RX425, RX520 printers."
I realise that the last one was rx520 and yours is an rx500, but they are from the same range, so they should be compatable.
I would give SVP (blankdiskshop) a ring on 08707 669166 or as they prefer communication by email, drop them an email at [email protected].
They are good co. to deal with.
Hope this helps.