This is one of two covers with this error that I know of. There may be others of course. Both are FDCs with a date of 1 MR 54 which was also the date of issue. This led to the conclusion that this error was present on the first printing run of these stamps. Whether it was present on subsequent runs is unknown.
This stamp is common as 10c was the rate for local mail but the error can be elusive. It has now been included in the latest 2014 Stanley Gibbons catalogue. The prices are £35 for mint and £10 for used. There is every prospect that the prices would rise subsequently.
From a part sheet, the position has been noted as the 10th stamp on the first row, R1/10 but possibly present in other positions as well. You can see my previous post on the adhesives by clicking the link here.