After extensive (Okay, about 30 minutes) research, it looks as though the fix is a relatively easy one. Of course all things are relative, but it looks as though its something I can fix myself with a bit of gumption (good Northern Word that).
Basically what looks to have happened is that the sail has stretched slightly, but the bolt rope down the luff has shortened. It seems a bit of a design flaw to use a rope that shrinks under stress as a bolt rope, but that's what happens.
The tell tale is lots of creases along the length of the sail. And guess what, here's a picture of my main sail:
|A very wrinkly and baggy sail|
While I'm messing with the main I might see if I can improve on things at the clew. There's a annoyingly long bit of string attached to one of the clew reef points and it dangles in your face at the most inappropriate times. I might have a stab at making something a bit more elegant with a small jammer pulley and a snap shackle.
I must admit that there's more to sails than just hauling up a bit of rag. Its interesting getting into the science of it all and seeing why one sail performs poorly against a different sail. One of my previous interests was aeroplanes and there is a fair bit of crossover between aviation and sailing. All you have to do is think of a sail as a wing stuck upright.
So a baggy sail will make a good slow speed aerofoil, but will generate lots of drag at higher speeds, hence why when the wind picks up, it just knocks the boat over, despite letting the sail out lots. A tighter sail will be a poorer slow speed aerofoil, but will be more efficient and generate less drag at higher wind speeds. If you have a sail that is able to be tightened for higher wind speeds, then you always have the option to let the tension out of it and make it baggy for low wind speeds. A baggy sail will always be baggy, you never have the option to trim it for higher wind speeds.
So, the next big job is sail tuning...
P.S. Notice I used sailing terminology like luff and clew. I'm getting good, I only looked clew up, I remembered luff, leech and foot from memory! I can also remember head and tack, but clew seemed to have fallen out of my brain. :-)