As you will already know, the vowels are a, e, i, o and u. Spelling in English is notoriously difficult: this is especially so in the case of British English. In many words, not all the vowels are stressed: this causes difficulty in spelling such words.
If you think of the letter a. It is pronounced very differently in certain words. Look at the following:
It's no wonder that spelling is such a big part of the English curriculum. Play this 11-plus quiz and see how good a British English 'spelling bee' you are.