I read this entire story yesterday and was so moved I needed to create an account to show my support.
I must say this is easily the best bit of fiction I've read in a long time. The way you plant things into your plot is marvelous and almost no detail is brought up at a later time. I loved how realistic Derek's character was. No matter how low he sunk, I never hated him. I always saw things from his side, which is what a good protagonist does. Even when he had hit rock bottom, all I could feel was excitement at how the plot had erupted and anticipation on how it would all come together. (After all, I did believe your promise of a happy ending.)
And to top it all off, the epilogue was the perfect icing on the cake. Even though I found myself crying (manly tears, of course,) it was fully satisfying.
I don't think Charlie NEEDS a sequel, but of course I wouldn't mind seeing more of these characters, especially now that our Hero has his memory back and is likely to start acting differently.