The UK has adopted the standard set by the American Food and Drug Administration agency (FDA) in terms of the maximum amount of microwave energy permitted to leak from a microwave oven.
This level is set a 5mW/cm2 measured at distance of 5cm from the potential leak.
Modern microwave ovens, when properly maintained and in good working order, do not normally leak microwave energy. However, in the event that a repair has been undertaken it is considered good practice by the repair technician to carry out a check for leakage to ensure that all seals are fitted and no safety aspect has been overlooked
Generally speaking, if a leak is discovered, even if it is below 5mW, it would be considered good practice to investigate, since it is unlikely to improve with age.