I thought I'd also post this up in here as I figured some other people reading this might get something from what I have written...
How far will do you think you will be cycling in training on average? How far is the charity ride?
IMO it's not just a case of trying to eat 1,000 cals and hoping for the best. You really need to make sure that you are eating consistently every hour during the ride and also you are eating a little more than normal afterwards (high protein + moderate-high carbs). Have you tried using gels? Those things are great, liquid food. Double check obviously whether the brand is vegan but normally they'll give you between 100-150cals so I used to get through a couple every hour when I was cycling and a sports electrolyte drink as well (better than water when you are exercising for an hour or longer). Then you can look at convenient, high energy food, bananas are great as are small food items such as the odd cookie here and there
I use a sports specific cookie, you probably can't get them where you are but you might be able to get something similar... http://www.powercookies.com/products/
Caffeine can be useful towards the end of the ride, I wouldn't use it at the start or middle for two reasons, 1) it's a diuretic, so apart from making you need to pee it will slightly dehydrate you 2) you'll get a drop off effect as the caffeine wears off, not what you want through a ride.
As soon as you're back home and off your bike get a protein shake into with some carbs (fruit is great) and then a proper meal about an hour or so afterwards. I'd keep the calories a little higher than normal for the remainder of the day as your metabolism and HR will be raised while your body adapts and recovers from the session; this can be anywhere from a couple of hours through to 12hrs depending on both the individual and the intensity/duration of the workout. Hope some of this helps, chur chur MF.