Haynes states the seal should go on the filter housing then fit the filter to the lid and screw down onto the seal.
Thank you for this. I've just discovered I had it assembled incorrectly i.e. element in first, then seal. I can see now how air that would be pulled in past the threads of the filter housing screw cap could be pulled into the filter housing rather than being blocked by the seal!
I've had issues with my car misfiring and eventually losing all power (tick over revs only, no throttle response, turn off and on again and away we go with no problems). Only happens after 70 miles or so of motorway driving only, not happening on secondary road driving. I guess there is a fair vacuum built up at that speed after an hour or so that could pull in the air, even more so as the filter begins to get dirty.
I will monitor same and post back.