I'm slightly embarrassed asking this because I pride myself on being reasonably good at training my dogs! That's the beauty of animals though, there is always one that tests your ingenuity!
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So I thought I would just lay this problem out for the very knowledgeable dog owners on this site to ask where I am going wrong please and sorry in advance for length of post.
I have a dog, a lurcher X'd with ?, who is probably one of the most intelligent dogs I have ever known; very kind and gentle, very agile, quite wilful, strong and whip smart. He's a rescue and had a horrible time as a pup. When he came to us, we couldn't actually touch his body , it was really sad. He cowered and shied away so heaven knows what had happened to him.
It has taken nearly two years to get him to the point where he will wear a collar and harness, be stroked all over, and be brushed (nearly) all over his body , be able to pick up his feet, and he is just about ok now with us brushing his head but you have to be very careful indeed not to spook him. We've never needed to clip his nails as we walk for miles every day as part of my work (stable dog) and he's finally ok with the toothbrush. He's evidently had an injury to his hocks in the past which has now healed up totally but the scar is still visible, like he got caught in something (wiring?) 
The main problem is that he is maturing fast and his coat is growing fluffier and longer and he's got some knots in it. He's the sort of dog that needs brushing every day but doesn't need regular trimming. I can stop the knots on his body developing with regular brushing. Unfortunately though some small knots are developing behind his ears that need clipping out and I cannot for love nor money get the scissors near him. I can brush his head now (although he still doesn't like it) but as soon as I reach for the scissors - even reach in the dog bag where the scissors are kept - he's off the table and up the stairs or out the door down the field. He doesn't seem distressed - he obviously is though - but he still runs like the wind at the sight of scissors. I've tried holding them near him and rewarding with tiny strips of steak and some chicken liver paté (his favourites!) but he's not particularly food focused and as I say, I can't even get the scissors out from across the room without him rushing off.
I've tried taking him to a professional groomer and literally could not get him in the grooming room. He howled and ran.
I tried a professional groomer coming to us and again, all fine until he saw the scissors, and the trimmer was even worse, then he legged it. It's like a horse bolting when he leaves. He crashes through anything in his way. There is no reasoning with him.
What do I do now? I'm worried that the knots will develop in to bigger clumps if I don't do something soon. I'd obviously rather not have him sedated. Or should I have him sedated and trimmed right down and continue with more desensitizing after that?
Thanks for reading this.