Ah yes it seem that you on your side of the pond have the same "knowledgeable" gunsmiths as we do on ours .The number of time I have seen this exact same thing . Discs can be very tight and every one on occasion has had some damage occur when taking them out usualy in the form of stretched key holes due to the disc being to soft and/or the key holes being to shallow . Not a bad idea to clean them out and deepen very slightly with an appropriate sized drill first if possible ,All my keys are about a foot long big handled and with a parallel for a spanner to give extra purchase .I also make them with longer prongs to get maximum grip. Many commercially bought keys are to short in overall length and prong length IMHO.
But that's still no excuse for incompetence .
