Well i want to start to take longer rides than the 45 min rides i do regularly, recently i've done a couple of longer rides (4 x 2 hour rides) but i want to know how to warm up. All i used to do was spend 10 minutes stretching, then get on the bike, ride slowish for 15 mins then start pounding the pedals but i'm sure this isn't enough, if i'm going for a 2 hour ride what warm up routine should i go through before getting on the bike, then when on the bike how hard should i push it when?

No pre warmup needed unless you're riding a short TT prologue where every second counts. Pre-stretching is counterproductive, especially on cold muscles. Use the first few miles of riding as your warmup. Let your legs, lungs and heart tell you when to increase the intensity. BTW it likely will be different for every ride. Some you'll feel like hammering from the start and others will suggest that you turn around and park the two wheeled puppy for some couch time If you insist on stretching do some on the bike while riding and/or afterwards but always after the muscles are warm and the blood flowing

I agree. There's really no reason to do any warm up. I don't even really warm up before races. They usually start slowly, and you're just drafting until someone makes a break anyway. I just hop on the bike, take it easy until my legs feel like hammering for awhile. Then, I know I'm ready to go.
Time Trials are different story, though.

thats good to hear, so what about a post ride warm down, do you take the last portion slowly to wind down? Then when you get home what routine do you go through, stretches etc.?

The last 1/2 mile or so is all I do to warm down and a 400' foot coast to my garage. I then sit on my front porch until I stop sweating unless it's real cold out. I never stretch - too lazy. The non-stretch thing is probably not advisable, so a little light stretching of whatever muscles you want to increase flexibility in would be prudent. BTW during my former racing days the event would end with an a** kicking sprint and the legs all charged up with lactic acid. In those cases a little light riding for a few minutes seemed to help.

Have fun.

You need to warm up as definite physiological changes occur when you exercise intensely, and a warm up allows your body to makes these changes less abruptly.

Usually, this happens pretty naturally - i.e., you simply go a bit slower when you start out on your ride - and that is all you need to do to warm up. Just ease into it gently for a few minutes, and you have accomplished your warmup.

A cooldown is pretty important if you have been pumping it pretty hard or it is hot. This keeps your blood from pooling. All you need is a few minutes of less intense riding.

Stretching cold muscles before the ride is not advised. Stretch warm muscles during or after the ride.

On top of what everyone else has said, there are also new studies out that question whether stretching is a good idea at all for most sports. You don't need to be flexible to ride a bike, although you do for gymnastics. Stretching tends to mask injuries to some extent and it appears to not help prevent injuries. The government something or other has started backing off of recommending stretching except for sports that need it. The article was in the last Runner's World or the one before it, I think.

Thank you all very much for the help, just got back from a ride where i set a new P.B for 40 miles - 2:10. Eased into it nice and slowly and same afterwards then after some light stretching i feel great!

Life is GREAT!