this turned out to be a combo of things, nipped over to PJ and sorted a proper set of plugs, bought a phase sensor and got a bit of advice, top blokes there indeed. However I noticed that the stuttering appeared again after "trying out" the new plugs. This seems to be when the petrol tank is below 1/4 , so i guess summat is wrong somewhere, for now the tank can stay above that.

thanks for the help

dan