Time Machine help

I read on this forum, while I was on holiday, it's best to have Time Machine on a larger HD than the one on your Mac. So I get home today and decided to transfer TM to my 2Tb HD and was stunned to see it held nearly 500Gb of TM alone!

What I want to know is: What is the quickest way to delete VERY old backup of TM and do I need it all before I delete most of it?

Thanks in advance for your answers,

Iain.