Having looked at the costs, going LED does not seem worth it in terms of my electricity costs - and, whilst, as a geologist, many of my projects were about environmental protection and I did my bit, I can't see any payback for me in reduced energy costs. So the tubes will stay.
I can, however, see the benefit of making sure the tubes are not on when they are not needed, so I've bought a PIR (with a variable time delay adjuster) and will now fit that. I would have fitted it on Sunday, but having taken the wiring to bits, I discovered I needed 3-core (not 2 and earth) for the PIR as it needs a permanent live as well as neutral and the load, so had to put it all back together again.
I now have the cable, the wife's out all day tomorrow, so that seems like the time to get the job done.
Thx for the advice given to my OP. Much appreciated. Changing normal house bulbs for LEDs is a given but changing the tubes seemed to be a different question.