It is a well known fact that many official overprints are faked to enhance the value of the original stamp. Quite often the seller will put the remark If genuine and it is then left to the buyer to decide, this phrase being often used when the seller knows very well that it is not genuine and may be the one to have printed or had the stamp printed with the overprint I  have noticed that many of this type of overprint has a midlands location which would indicate a straight forward practice of deception. Buy if you must but to be more certain you have the genuine article then buy used with a dated cancel.