Across the board, dictionaries suggest that percent, written as one word, is American English, whereas per cent, written as two words, is British English. The European Commission (Directorate-General for Translation) English Style Guide concurs, although it is not prescriptive and notes that per cent is normally [blog author’s emphasis] written as two words in British English. Wikipedia even goes as far as describing the frequency of use of the two-word form in British English as “sometimes”. In any case, the online free dictionary suggests that the use of the two-word form is diminishing. No evidence is given for this statement, but it would be in line with the (in many case entirely ‘sensible’) trend of compounding. Not to mention the fact that, from a German Prozent perspective, the one-word form simply ‘feels more natural’.