I would personally recommend Magnesium Ascorbate as a form of Vitamin C as the magnesium content is extremely beneficial for the heart muscle. I purchase mine from another UK company (I live in the UK too) called BioCare through my local health-food shop. This is the best product I have come across..............
Well, believe it or not Vitabiotics is a British Company with an outreach to the States.
This is the link, based in the UK where you will save a lot of money on shipping.