Many thanks for your reply - I now have replaced the fuel filter and have tested the car on at 70mph with no juddering or engine cutting out or loss of power as before.
However, now the coil light is flashing on and off when I attain 70mph and continues as I drop down in speed until stationary. I turned the engine off and restarted when no coil light flashed. Drove up road again and coil light started flashing at 50mph again with no loss of power - increased speed up to 70mph still with no loss of power but coil light continued to flash.
Please note the coil light did not flash in this manner before fitting the fuel filter - it only flashed when the engine cut out.