Advice by West Yorkshire Trading Standards is that it is best that to have your scales professionaly maintained and that they should be officially stamped as accurate. I quote:
"Use for trade means any determination of weight, length area or capacity upon which the price charged is calculated.
Virtually all trade equipment is required to bear a stamp or mark to show that it is suitable and accurate. The stamp comprises a crown, a number. & the year
Some weighing equipment may have an EC verification mark, which is the “CE” mark and a black “M” on a green background
Weights and scales intended for kitchen use are not stampable and thus cannot be used for trade purposes.
The Crown stamp can be found:
• on the bottom of a weight
• by the 1cm mark on a metre stick or tape measure
• on lead plugs on a weighing machine
The EC verification mark can be found:
• as stickers applied to a weighing machine
• printed on the chassis of the weighing Machine
Generally, it is wise to have weighing equipment maintained by a competent
scale firm.
If an officer finds the equipment outside the permitted tolerances he will reject it by obliterating the stamp with a sixpointed star. It then is an offence to use the equipment before it is repaired and restamped.
It is also an offence to use equipment which is seriously inaccurate."
Presumably this is in addition to any offence of selling over or under weight goods.