we have a meter, two of us in the property, and we water the garden in Summer too, our bill for a year was £248, sounds like you do have a leak, there are often leaks and faulty meters too, my daughter left her property last year and it was unoccupied for 7 weeks, she got a bill for £239, there were no leaks inside, and by the time to leak was mended by the water board the bill was about £700, which because it was the house side of the meter, they could have made her pay, but thankfully they did credit her with the cost, best to get your meter and pipework checked asap..........