This is the one I stock where we work...Delia rates it so I guess it must be ok.Hate the stuff myself but we sell masses http://www.tracklements.co.uk/
theonlyone is right about the Polish horseradish and beetroot relish - called cwikla (pronounced chfeekwa) - deli shops and increasingly in supermarkets. For horseradish on its own, look for jars of "chrzan" (Polish word for horseradish). It looks quite dry as it's not mixed with anything but it has a hefty kick. Worth looking for.