It is not necessarily illegal to have a dog's tail docked.The Animal Welfare Act 2006 s6 bans the practice with exceptions. Two conditions must be satisfied, above all.First a vet must certify that the dog is to be a working dog, within a narrow definition of 'working' (which would includes dogs for such purposes as the defence of military establishments) Second the dog must be "of a type" which is certified under "regulations made by the appropriate "national authority". Act s6(5) and 6(6)Can't tell , as I've not seen the regulations,whether a rottweiler is "of a type" certified ,but the RSPCA will assist. It's really a matter for them to consider and proced with, as appropriate..It ought to be hard for the person having the docking done to satisfy the conditions ( I fear it may not be, in practice!)