You need an 8 pin switching headphone jack, I found some on Ebay that were a perfect fit internally but had a slight wider socket so I had to file them down.
The headphone jack seems to be a bit of a weak spot, I bought 2 GTs recently and both had bad sockets. If anyone knows of a source for exact parts, I'd like to know (Keith? you are a font of knowledge on this console aren't you!)