Hi W - not done this one - using any cola makes me shudder anyway even though it is a good ingredient for cooking beef, but I do know that there are recipes that use a veg oil or buttermilk in the mix which will help with the moisture content -
have you checked the proper settings on your oven by the way? - the simple test is make a couple of victoria sponges and cook on upper and lower shelves - they should rise together, and this will tell you if the thermostat is accurate, as you may have the settings too high - hence the dry result this time...
Quick trip to the dark side - or the Coca Cola Company as its also known...website gives you this recipe (pdf file) -
http://www.thecoca-colacompany.com/heritage/pd f/RECIPE_colacake.pdf wehich might be an alternative to try - young Mr M doesn't always get the best results from his recipes, as a trip to his fan club - sorry the BBC Food site - will tell you..
Hope this helps...