Invoice for £49.99
I got an email/invoice from Apple even though i haven't purchased anything
You are sure that it was a genuine email from Apple and not a phishing attempt trying to get your account and payment details ?
Phishing and similar emails : Phishing & Other Suspicious Emails
Phishing emails : Identifying fraudulent "phishing" email - Apple Support
Genuine iTunes emails : Identifying legitimate emails from the iTunes Store - Apple Support
Viewing your account's purchase history : See your purchase history in the iTunes Store - Apple Support (on your iPad you can view the last 90 days purchases via http://reportaproblem.apple.com)
If it's a genuine email and it shows on your account's purchase history, and nobody else has access to your account or devices/computers, then contact iTunes Support : https://support.apple.com/contact . And change your account's password http://appleid.apple.com - after changing it : What to do after you change your Apple ID email address or password - Apple Support.
If it is a phishing email you could forward it to Apple : reportphishing@apple.com , and then delete it.
If it's a phishing attempt and you clicked on the link in the email and entered your account id and password then change your password immediately