Unfortunately my Eheim was bought used. So im pretty much screwed when it comes to the warranty.
Not really, It's a known issue and as the 2071/3/5 range was introduced July 2009, the oldest ones are still under warranty until July 2012. Speak to John Allan direct, perhaps not mention you bought second hand unless asked, just focus on the leaking issue.
I bought 2 2073's for myself and a 2075 for my friend, lost the receipt so thought I was up that certain creek with not a paddle in sight. Called my retailer where I bought them, he spoke to John Allan who confirmed the problem and without any quibble, three new heads were sent out to me. I said I wasn't aware my friend's one was leaking as yet, but was concerned it would do at any time, didn't even argue, just said they would replace all three.
Aparently people were not returning the old heads so they started incurring costs so now they will either ask you to return the head and they will send a new one in return, or you can pay a deposit, they send a new head and then you send the old one back and then they refund your deposit. The only cost to yourself is the cost of sending the old heads back, but the only other option is to go and buy a brand new filter, so it's a lot cheaper to get them replaced under warrantly.
If your's is not leaking, I'd still speak to them and raise your concerns if you bought new before Aug 2010. Once you get out of the 3 year warranty that's it, you lost your chance to get it fixed free.
Just thought I'd post this up as I find it annoying when you have a problem, search the net and discover loads of people all with the same issue, but no answers to a definitive fix.