Hi NJK, San Javier is very easy. Usually rwy 05R (05L for the Pilans) is in use and this side is very open, you'll figure it out. My preferred spot is along the road to the campsite (Mar Menor) on the soutside of the threshold in the morning and next to the holding point on the other side in the afternoon. Nice head-on and side shots on the taxiway are possible. Just keep an eye on the approach and the car (or bicycle, as many do) ready with its nose pointed in the direction of the sea, as you need to race to the other side when something interesting is on approach.
There are always some local spotters (elderly men with radios and ancient cars) and they might know something (sometimes, no guarantee) about expected visitors and movements. Recently I have seen quite a few shots of visiting fighters from local photographers, mostly EdA Eurofighters and Hornets.
Landing shots are good from the start of the hike trail Paseo Sofia, where the road to the coast bends north along the runway threshold. There is an information sign and some benches to stand on.
You can leave your teletoeter home (100mm suffices for the holding point!) but bring a small stepladder
Veel plezier!
Bart
PS The food at the adjacent Mar Menor campsite restaurant is quite OK, although the service sucks.. But the vicinity of a cold drink and proper facilities so close by make it bearable on quiet days.