As a general rule, all birds, their nests and their eggs are protected by the Wildlife and Countryside Act 1981, and it is, therefore, an offence to damage or destroy an active nest or prevent parent birds access to their nests.
As such, should you suspect that you have birds nesting or causing damage in your attic space, legally we are unlikely to be in a position to have them removed, or to arrange for access holes to be filled, until the young have left the nest. This has usually happened by September or October, however, pigeons can nest throughout the year so it may only be possible to remove them if it can be shown that this is necessary in order to preserve public health.
If you wish to deter birds from nesting in your roof, work to deny access must be done during the winter months. If you have had issues in the past we would recommend checking your building between October and February and asking for any holes or gaps discovered to be cleaned out and sealed as appropriate.
Further information on this subject can be found on the RSPB website.