I agree with everything Paul Ell (above) has written. Like Mr. Ell, I found that the two main cables (apart from the green/yellow earth) from my electricity meter were grey; according to the instruction leaflet one should be red, and the sensor attached to that. I attached the sensor to the right-hand cable, and it was correct. Incidentally it is a very tight fit, a snap-shut fitting which only just fitted the cable; I assume the designers intend this.
In general, I think the booklet could be improved upon; it is in very small print and did not seem particularly easy to understand at first reading, although there is also a fold-out 'idiot's guide' included with the box. Also you MUST have a really small cross-headed screwdriver to hand (like a jeweller's) to get access to the battery compartments on both sender and receiver units. (Batteries are included thankfully).
Having said all this, once up and running, I have found it invaluable. You can set the display for Cost, Energy (kW) or Greenhouse Gasses. There is a problem with the Cost function, as if like me you are on a tariff which decreases once past a certain kWh useage, the Owl cannot be set to automatically adjust, although it can be set to adjust to different tariffs at different times of day. So I have mainly used - as I suspect most people will - the Energy display. This shows how much energy is being consumed at any one time and is a real eye opener. I found that a 3 bar electric fire which had all the bars turned off was still using approximately 700 Watts! This turned out to be a convection heater which is built in and which I had, until now, assumed to consume negligible current. Also, like another reviewer, I found that despite turning off/ unplugging everything in the house, there was still over 100 Watt consumption taking place. Is that the meter itself.....?!
Seriously, it really is worth every penny, and I am sure will rapidly repay its cost given today's energy prices.