after a week of teeth gnashing i have replaced the universal lambda sensor with a correct denso one from eurocarparts, £77 well spent, fault now cleared, do not buy a universal sensor, the heater band is too wide for jags, and will trigger the fault light, just to clarify p1646 is generated by the sensor in the manifold at the back of the engine, bulkhead side, and 1647 is from the upper
sensor in the manifold at the front, radiator side of the engine, once again, DO NOT BUY A UNIVERSAL SENSOR!!!!!