The police have to serve a summons on the defendant within 6 months of the date on which the prosecutor (normally the CPS) certify that they have collated enough evidence to bring a charge.
This does not mean that the police have 6 months from the date of the offence to charge the accused.
My advice to anyone who has received a summons more than 6 months after the alleged offence is to request a copy of the signed and dated certificate of prosecution from the Police/CPS
http://tinyurl.com/6fxhgq