The number referred to in the code is the tyre's load rating. 83 is rated to 487kg, while 87 is rated to 545kg.

In respect of the MoT, testers are obliged to check that the load capacity is in accordance with the minimum requirements and is sufficient for the axle's presented weight. Not meeting these requirements is a failure on both counts. You can check the axle weight of your vehicle on the axle weight plate, which you'll find either under the bonnet or on the passenger door jamb. The combined load rating of your tyres should equal or exceed this figure.

In terms of performance it is unlikely you will notice a difference, but we would advise returning to the tyre fitter if they do not meet the requirements. Similarly your insurance may be invalidated in the event of a claim if your tyres do not meet the requirements.