P0421 problem for Shogun Pinin

Hi I have a 2003 Shogun Pinin which keeps producing the fault code P0421 - Warm-Up Catalyst Efficiency below threshold (Bank 1). Both O2 sensors have been replaced and the exhaust emissions are within the required legal limits. A Google search suggests this is a recurrent problem and a fix is available using spark-plug non-foulers (see http://www.club3g.com/forum/showthread.php?t=55284) . Unfortunately these devices seem to be unavailable in the UK. Can anyone help, either with the problem itself or with sourcing the non-foulers in the UK?