How much are the 40-year-old stamps that my late father collected worth?

In 1973 my father bought from the local Post Office a number of stamps. They were European Communities 1973 stamps and were sold to him the day before they were due to be issued.
Having noticed this, he posted ten letters to himself and they are post marked 'Slough, January 2, 1973'. He probably thought he would return himself a healthy profit.
He died many years ago, so after 40 odd years it’s time to find out. Some of the stamps have been franked several times possibly to void the fact of the Post Office error. A.S.

Valuable? My dad was sold some stamps in error 40 years ago, are they worth anything?

Antony Grodecki, group commercial director at stamp investment firm Stanley Gibbons, says: It was not unusual for stamps to be used before the official day of issue although it was in breach of Post Office regulations.

Local offices generally received the stamps before issue date and sometimes sold them early either in error or as a favour to a customer.
People who bought them early quite often then chanced their luck and posted them before the issue date and frequently these were not spotted by postal workers.
They are essentially novelty items and as such do command a value, although not enough to retire on! A dealer would generally sell such items for around £10 each.