led floodlights to replace elderly halogens - will PIR work?

Depends. If there is an audible click from the IR detector when triggered then it is likely it contains a relay and it will work fine. If it is silent, then there could be some doubt as some electronic switches need a minimum load.

You could try a low power filament lamp such as a oven/ fridge bulb which are 15watt as a test.

If the PIR has its own power supply with both a neutral and live, then chances are it will work since its probably just a relay doing the switching.

PIRs that are designed to replace light switches and run without a neutral probably won't work (since they need to bleed their operating current through the lamp)