So sorry to hear about the fall, Robin - that's awful. Not sure how suffering with a bum knee will help your art, but I assume your recovery period spent with a camera will yield an answer to that question.
Here are my thoughts. First, as to your questions...
1.would you clone out blue sledge toward upper left corner
To be honest, I hardly notice it. Could be my monitor or my eyes, but it doesn't appear too blue and isn't distracting me.
2.have I included too much of the tree trunk
Absolutely. Unless it's the tree you wanted to showcase, in which case you have too much depth of field.
3.have I increased the colour too much
Not for my monitor. White balance seems a little off for me, and while blues and reds of the jackets and boards may, in fact, be oversaturated, it's difficult to see with the image you've uploaded.
As you obviously have noticed, this photo wants to be about what's going on behind the tree, but we just can't get there with the tree in the way. I can't tell if it would have been better, but I believe had you moved left and squatted a bit you could have used the overhanging branches to add something to the overcast sky without them intersecting the horizon, or just gotten rid of the tree altogether. The other option would be to make it about the tree, bringing your DOF down so that the activity behind the shot is blurred, in which case you want to step back and grab the entire trunk. I think you also have too much to the left of that blue sledge you mentioned.
It's just hard to tell what the picture is about. I know - I've taken a lot of these. The idea is there, but the execution got lazy. The idea of snowboarders framed by the tree is a good one. But you needed to move around a bit and figure out what you wanted to show and how you wanted to frame it, which can lead one to be rather conspicuous, and that's something I had to learn to get over. Great photos don't always come from 4 to 5 feet above the ground.
I also find it a little dark and over contrasty.
Not a lot I could do with the jpeg as-is, but I wanted to play with it (all I have on the work machine is Elements 9) - hope you don't mind. I did a different crop, enlarged it a bit just so it's not too small, and played first with the colors and contrast (your reds are WAY oversaturated) and came up with this one edit. Still wasn't crazy about it, so I thought I'd try a black and white, to which I applied a magenta filter as it seemed to bring out the details. I like a little better, but would have preferred separation between the horizon and the branches.
