How do I turn this off?!

WO lettering is cast into the lid which stands for Wash Out,all covers are normally stamped as to the purpose they are meant to perform.
It also has STW cast into it which is for Severn Trent Water.

 

I can't really see from your photos but it may be similar to the type I came across in Cheltenham. If it is, then the whole thing will twist 90 deg from on to off. I had to ring ST to ask btw. Sadly my memory's not what it was, and when it was it was rubbish, but I recall this particular one was stuck so we fashioned something out of a bit of timber to act as a key. Once we moved it, it was bloody obvious how it worked but, there again, with hindsight most things are.

 

I've been wondering if it might be something like that but couldn't find anything on admittedly limited research of the interweb nor did I have the courage of my convictions to suggest it. Wuss.

 

Softly, softly gets my vote. You might waste ten minutes sussing something out but you will waste many hours clearing up the mess of hasty action!

 

Here the aquadis company site, not much info.

http://www.autometers.co.uk/products/view/156/itron-aquadis-cold-water-meter