Not sure if it is a floating deal (see below), but when you buy a Boots Meal Deal, you get an extra bottle of water free.
Can't come in wrong if the weather is good.

I say floating deal, as when I was in the Silverlink store early last week, they had the offer on then - with advertising literature on the fridges.

When in the Arnison Centre store earlier, there was no promotional material, but when I was given the free bottle at the till - I mentioned how I thought the offer must have ended, with there being nothing about promoting the deal. I went on and explained that I had had a free drink from Silverlink last week.
The sales assistant responded and stated the offer had just started that day and that I must be getting mixed up!