I ended up using Milton tablets last time because it was all I could get last minute.
They’re for sterilizing bottles and equiptment really, so very strong and you only need a tiny fraction of one tablet – there’s a guide to the amounts to be found somewhere on their website – and it’s available in most Supermarkets, along with all the baby products.
edit:
Add half a teaspoon (2.5 ml) of Milton Sterilising Fluid to every 5 litres of drinking water. Let it stand for 15 minutes before drinking
The tablets are obviously easier to carry, and you can roughly work out the quantities by comparing the fluid against the tablets.