Thanks everyone for your input.
I participated in another SAREX on Saturday and the off-frequency technique worked perfectly yet again. On-frequency we defined a circle a few miles across, at 1000 feet AGL. We then started searching in the centre of the circle, using a 50 kHz offset and we picked up the signal within half a mile of the target ELT. It was fairly easy to find since we knew it would be on or close to a road. We were cheating a little bit!
9Aplus, thanks for the link to the SDRs. I’m going to look into that.
I am now a fully qualified Spotter and I’m on the list in case of a real call out. These typically happen about twice a year.