[DanBC]

"Dear $NAME,

I am writing to give my $LENGTH_OF_NOTICE notice as required in my contract. My last day of employment with you will be $DATE

Please let me know if there is anything that you'd like me to focus on in my last $LENGTH_OF_NOTICE months employment.

Your sincerely,

$YOUR_NAME."

This is all you need. You don't need, nor want, to go into the reasons. It's probably best to not mention the reasons even if they specifically ask; just tell them that you got a better offer somewhere else.