'E' stamps are for sending a letter (up to 20 grams) to any European destination. They've been around since 1998, but Royal Mail discontinued them in 2004 as people were confusing the 'E' to mean 'Euro'. They are still perfectly valid to use, with an equivalent value of 97p ( 2014 ) which is the European standard rate of postage. Stamps with E are effectively inflation proof and will go up in value as the 20g price to Europe increases each year.
Many thanks, dannyk13. I tried the telephone number on the website which you indicated and got a very hesitant reply that the value is now £1.25 each. Sounds about right but I will have to call in at my local post office to be sure.
Many thanks again for your help, folks. I have now spoken to Stanley Gibbons ( who else ? !) who positively confirm that the face value of "E" denominated stamps is now £1.25 each.
I bought a stamp this afternoon for 'Europe up to 20g' and it was £1·35.
Just checked on Royal mail website and that confirms £1·35.
https://www.royalmail.com/price-finder