Hello and welcome,
I hope you love your K100 as much as I love mine! 6mp is plenty for my needs and the camera is dead easy to use, especially if you have old manual lenses like me.
As a bike enthusiast I have to say what a great shame it was to hear the news about Robert Dunlop. What a family of true sportsmen.

Rob

Clarky has developed at great length the MBD ( Milk Bottle Diffuser) which I have tried and it is pretty good. Try a double layer as well as a single. The locating mechanism is equally exotic in my case: a rubber band!
Seriously though, make sure there is a small gap (approx 1 cm or more) between the flash and the diffuser so that heat can escape from the flash unit. With repeated discharges you could wreck your expensive flash gun.
I wouldn't strap bubble wrap around tight like that.

The 10-20 Sigma lens is sharp, but a few have given problems. This one appears good as far as I can see. Problem ones are identifiable from the images posted here.

The sharpness you mention is rare with this lens and your pic looks OK to me. Correcting the distortion is done by using the "shift" adjustment to straighten up the people etc that are leaning in to the centre of the picture.

I agree. You can also look up the track with cars going away from you, although some things don't look as good from the arse end!

If you can position yourself so that you can look back towards a bend so that the cars are coming towards you, then all you have to do is to manually focus on the point where the cars are just about pointing straight at you, the just click when they are at that point. A tripod and remote release is great for this but not vital. Of course if the light is low you might have to use a high ISO. If you want to practice panning go out to the street and practice on a few passing cars. They won't all be sharp, but some of the ones that aren't could give some dramatic effects.
For the paddock the 18-55 should be absolutely ideal. It is an under rated lens in my opinion and gives nice sharp results.

ChrisA,
What anactodal evidance?

I don't think it really matters who actually makes the lenses, as long as Pentax apply standards to the manufacturer that the product meets the expectations of the consumer buying a lens with the name Pentax on it.
If the quality is below what you would expect from the name/price combination then Pentax would be making a grave mistake which would weaken the brand almost at once. So, to me, if the quality is good it doesn't matter who makes it.
Badge engineering has been going on for decades in all areas of consumer products and most people have never either known or cared as long as they are happy with the product.

Lleyn from the East

If you are going to Cornwall, take LOTS OF MONEY. They live to rip you off.