Can't get into DFU mode

Hi all, working on iPhone 6 which arrived dead. Tried it on iTunes when I first got it, wouldn't connect in normal mode but recognized in DFU mode but stopped when the process got to 'waiting for iPhone' point. Some weird voltage behaviour as well, so changed Tristar and phone now boots up fine, only to find passcode on it which the seller doesn't know(!). Not recognized by iTunes but I thought no problem; I'll do a full restore in DFU mode to restore the thing completely, but whatever I try I can't get DFU mode to start. Although I've done this successfully many times before it's just not happening, so I've tried all sorts of weird and wonderful key combinations and counts etc but nothing. I then let the battery completely discharge and tried it then but still no response, so I've got a great-looking 128Gb iPhone 6 working fine but can't reset it! Really grateful for any help to get me over the line with this one...