Father Christmas somehow knew that I needed new and lightweight water purifying equipment, and he (my wife, actually) gave me a SteriPen Adventurer, which is smaller and lighter than the original SteriPen, as well as being more affordable, particularly if you shop around on the Internet.
For anyone who's not familiar with the item, it's a little, battery powered device about the size of a small-to-medium torch, that uses ultraviolet light to treat water by "disrupting the DNA of microbes" simply by pushing a button that gives a timed (48 seconds) treatment, with indicator lights.
I have two minor concerns about its use:
(1) It's not going to remove solids and gunk (sediment, vegetable matter, etc) like a filter would, nor chemicals, pollutants, heavy metals, etc. Naturally, I do not intend to carry extra weight in the form of a filter. On the other hand, maybe this is not worth bothering about, because water purifying tablets don't do anything about these either, and I used to use Potable Aqua on trips for several years.
(2) Of slightly more concern, it doesn't disinfect above the surface of the water in the container (flask, billycan, whatever), so droplets of water on the sides, or the rim where you drink from, could still be contaminated. With Potable Aqua and the like, you are instructed to loosen the cap of your container and shake it after treatment, so that the water goes over the lip and screw threads.
They say two (or more) heads are better than one, and I'd be interested to read other forum members' thoughts. I'm probably worrying about nothing. After all, there are plenty of walkers and backpackers who will happily drink water directly from mountain streams without treating it, but I have a sensitive stomach and I'm not prepared to take any chances. I still haven't forgotten the collywobbles I once got in France after brushing my teeth in a little stream.