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Diamond Back Response. Shimano Deore Rear Derailleur
bike is 2 yrs old. i don't use it very much, just short rides on easy paths in the summer months. in this time I've snapped 5 chains, this time completely ripping off the rear derailleur. I now have a theory that maybe the bracket that connects the bike to the derailleur was bent when i had the bike,(hence the weak point where it snapped) putting extra strain on the chain as it twisted slightly getting between the back cassette and the 1st cog on the derailleur. I've looked at getting a new derailleur, but they don't come with the bracket, so is the bracket classed as part of the bike? and if it is does it come under the lifetime guarantee of the bike? what is this bracket called i can't seem to find one?
This is an alloy replaceable drop out, deliberately made weaker than the frame so that this breaks and not the frame (alloy frames are difficult to repair), your local bike shop should be able to get one for you if you take the old one in for a match.

Yes, they can get bent in transit, or any time the bike is dropped and it can be difficult to spot unless set up by a skilled mechanic, on line buyers beware!

This a typical one from Raleigh: Outs
What I don't understand is how you have broken so many chains!!! are you fitting them yourself or a shop? are you using split links or riveted chains??
riveted chains. it was under guarantee so it went back to the shop the 1st 2nd 3rd 4th time. repaired 1st 2nd and 3rd, new chain the forth time. A more expensive one apparently. i didn't use the bike at all last summer because of bad weather, so although the new chain was a year old it was unused. Don't think i've ever done more than 30 miles without snapping one! My girlfriend has a cheap £100 bike, never a problem!
p.s i'm not a fatty!
The thingy that connects the derallieur to the frame is called a Mech/Derallieur hanger, and most companies will not cover it under the lifetime warranty for one simple reason:

They're sacrificial; they are deliberately built to be weaker than the frame so that when the mech takes a serious knock the hanger bends/breaks instead of the frame. There are several different designs, only one of which is compatible with any frame. Take the broken hanger (it should unscrew from the frame using a 5mm(ish) bolt, same with the mech) to your LBS and they'll sell you the correct one.
A bent derailleur hanger would not cause the chain to break. What could do it is if you are "cross chaining" a lot (going from big chainring to big chainring or small to small). But even that is not so likely. If you are shifting under strong load (pushing hard on the pedals while you shift) you could also cause this. Is it usually breaking while you are shifting? Still, that's a lot of chains to go through. Is it the same mechanic working on your bike every time? Got to wonder if he knows how to put a chain on.
The chain isn't breaking, it is coming apart at the riveted joint?
I would say that the mechanic isn't using the riveting tool properly, it is essential that the rivet is through both sides of the plate by an equal amount, and in any case, modern, narrow chains are not intended to be fitted by riveting, that's why they supply a clip link, you only use the tool to remove links to fit the chain to length, then use the clip link to join it, making sure that this is fitted properly, of course.
Have a look at this:

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