Earlier this week, when conditions were quite good, I was scanning up and down the 2m FM channels to see what was coming through. The receiver picked up some activity on 145.3375. I could tell that the signals didn't have much to do with the tropo opening so I moved on, but filed the frequency away for future reference.
Once the band had returned to normal, I came back to 145.3375 to take a look. It became clear that there was an Internet gateway, MB7IDM on the frequency. A quick look at Google revealed that the gateway was located at Calvert in Buckinghamshire, run by Gary, 2E0RHR.
For the last couple of days, I've been listening to the gateway off and on, but it wasn't until that I got around to plugging the frequency into the rig, along with the CTCSS frequency (103.5hz) that I found I was able to access the gateway.
Coming home this evening, I was able to have a contact with Will, MW3WSC in Porthmadoc through the gateway, which was great fun. Hopefully I shall be able to use the gateway more regularly now that I have figured it out. I may even be able to put the DTMF keypad to good use and work out how to connect to different nodes.
Thanks to Gary, 2E0RHR for putting the system on.