I spoke to my GP a lot about the vitamins, and she said that there really isn't much difference between the prescription-based vitamins and those you can buy over the counter. Some will be a bit stronger than their OTC counterparts, but you can simply take a higher dose to make up for that. The primary reason they even do vitamins on prescription is because it works out cheaper for the patient. So if you are on a life-long regiment, they do this for you to reduce the costs.
With that in mind, if you really can't stand the Forceval (and like Hannah says, you wouldn't be the first one), just try anther multivitamin instead. The OTC variants I was recommended were Sanatogen A-Z and Tesco Multivitamins.