After a bit more research I think I am looking at a PIR or a sharp distance sensor. Pir seems cheapest but I will need to baffle it to make more directional and I have read that it may be affected by sunlight? Distance sensor up to 5.5m will be fine, are there any down sides to these? Or back to the laser pointer and diode, gone off this idea as the beam may be visible in photos?
Any comments or suggestions appreciated.