djglover, sok's comment about randomised control trial seems to still be relevant even though you are saying "The control group is was gathered after the trial started, but it is made up of people who are in the same age and affluence groups, property types and consumption strata and towns, so the groups are fairly robust I think." Did the group with the "intervention" select/volunteer to add it? You might argue that they were more disposed to "energy saving" anyway than the control group who for whatever reason didn't volunteer.
However, from a commercial / career point of view there may be no harm in providing him the data, written into a report which says, "here's the data, but here's why it might be wrong to read to much into it".
If you have all the data you should be able to extract the %age of households who change consumption by 1,2,3% etc and likewise for the control group? You should also be able to take the %age change and standard deviation for the populations as a whole and estimate the number which change by a particular factor, that will be weighted to the whole population so if you have enough data individual housholds wont affect it. It a bit of a fudge, but thats the sort of thing your "stakeholder" is looking for. If you simply tell him the trial can't tell him that he will blame you for designing a bad trial.
I would include a remark to the following effect, "This is a simplistic statistical analysis, and robust detailed analysis could only be performed by specialist expert statisticians. The Company would be advised to seek such independent advice before basing financial, investment, legal or strategic decisions on this information."
When I caveat stuff like that it usually pisses people off, but they don't really have a good argument, if they want robust stats employ a statistician, if they want rough and ready numbers ask me to do it, but don't complain if down the track someone says its all a bit ropey.
I think what you need to do though is give him enough of a hook that he believes its worth engaging the professional consultant (assuming that is what you want) rather than simply saying, I don't think we have the data to answer that question.
EDIT: If the numbers really are this close "But I don't think I can say "40 people changed their consumption by 1% because of the intervention", because 38 people from the control group have also changed by 1%!" then I doubt you have enough data to say that there is a statistical difference between the two groups. i.e. if you repeated the trial that they wouldn't be the otherway round. That may not be the answer your senior stakeholder wants - and that is a much harder story to tell!
