Xbox 360 Warranty Question?

Please read below..
Hi,
I have an Xbox 360 with 2 red lights (overheating) since I sent the console in last year for repair with a 3 red lights problem. I have not used the console for about 5 months due to no internet in my flat.
It has been going to 2 red lights on and for months (mainly when it is online) but recently it now refuses to go on. It is NOT hot in the slightest, probably meaning a hardware problem. I spoke to a nice guy on the phone earlier who said he would fix it under warranty (not sure if he was allowed to) and I would send it in. The problem (and purpose) in this question is that when I got it back, I removed the warranty sticker so I could hoover the dust out of the console and vents, meaning they probably wont fix it. I have read posts online by people who say they still do it anyway without the sticker but not always.
Is it worth sending this in or will they just send it straight back to me and bill me for P&P?
I have not opened the console for hacks just for cleaning. I was thinking on writing them a cover letter explaining this and hope for the best? Or should or say that the last time I recieved it, they did not put one on here or the guy on the phone said it was okay? What do you guys think?
Is there any way to obtain a new microsoft warrenty sticker?
Sorry for the long post, just want to get the facts accross properly. Cheers.