Every bodies wieght goes up and down during the day, they do recommed that you only weigh your self once a week at the same time wearing the same or simlar clothing, guess this is advice from salter to avoid this type of issue,lol
I am a big guy and weigh more than the avaerge set of scales can manage so I went to a shop specalizing in scales, they sold every thing from small spring scale that a fisherman may use to scales to weighing trucks, (this was in auistralia but im sure he uk would have something in the yellow pages).
I purchased a set of medical standard digetel scales that can do eith pound or kilos and has 5 save buttons so the whole family can store he weight they were the equivelent of around 120 pounds 5 years ago, just need new batteries every year and they have never missed a beat, but still have lost much weight