Hey Ian,
I bought mine on some internet tool website. It's no Snap-On, but it works to get the new sensor tightened up. I just googled "slotted socket" or "sensor socket" and found one.
If you want, I can Priority Mail mine to you. You'd get it in Friday mail delivery. Just mail it back to me via regular US Mail when you're done.
P.S. if you make one, use a deep socket so that you don't have to put too much of a bend in your sensor wire when you slip the socket onto the sensor head. You'll see what I mean when you do a test fit.