I have to admit I can never remember! So half the time I spell it MANGOES (which I think is the technically correct version) and half the time MANGOS! So at least I'll get half marks!

The correct way is "Mangoes". However, several words which finish with "o" have the plural form with just the "s". I recommend you to check this link in order to study this kind of spelling rules ;) http://www.oxforddictionaries.com/words/plurals-of-nouns

U.S. usage is almost always "mangos".

Most of the nouns ending in -o form the plural with -s. When you have a vowel before the ending -o, the plural is always with -s: radio-radios, studio-studios, zoo-zoos When you have a consonant before the -o, there is not a rule that helps you. You already have to know how it's the plural form. Most of the words use only the -s but in some cases the plural is made with -es (hero-heroes, tomato-tomatoes..). Sometimes you can use either the -s or the -es without any distinction, the two forms are acceptable (motto-mottos-mottoes, volapcano-volcanos-volcanoes, halo-halos-haloes). Your word, "mango", belongs to the last category, so you can you both forms to make it plural (mangos-mangoes).