I know this is a stupid question but my ex girlfriend has my details on her phone so all she needs is the last 4 digits and the security code to top up and she knows these, know I've googled and couldn't find a way to remove the card from her phone is this possible and if so how?

Maybe it's a bit harsh but instead of trying to get your details off her account (which is probably quite difficult unless you are named on her account to be able to access it), why not just dispute any charges she makes to your bank / even report it to the police as she's clearly doing it without your permission?

Or change your card.

Report the card as lost/stolen to your bank

Yes it is. Contact the customer service dept of your bank and request that they decline transactions to top-up your girlfriend's phone. If your card is used to top up more than one phone on the same network, you might need to provide information about recent top-up transactions so they can identify the correct one to 'block'.

Thanks for the replys guys