It is a strange one as I've never seen the price dip below £5 before. Don't mention a drought - there'll be hose pipe ban imposed before you know it
Thanks for the feedback Shambles, I've got a dashcam too so will avoid applying to the windscreen. I'd also heard it's more difficult (if not impossible) to repair a chipped windscreen that has had Rain-X applied. The plan is to apply this to the side windows (and possibly the rear windscreen).
I've previously used Rain-X on glass shower doors and the results were brilliant - easily lasted a couple of months before I needed to re-apply. Much easier to clean too.