Ah, people are catching on. It is the drafts you need to cut out first, which is greatly reduced by having less opening to the ceiling (i.e. less than 5% as opposed to about 10%). Also, with a single fitting if everything else is well done then there is little draft as the air has nowhere to go, but with multiple fittings and ~10% opening the draft easily flows between fittings as you get pressure differentials.

Next you want to improve your insulation with abutted insulation instead of say 5cm clearance around each fitting.

Like all scientific data you need to interpret it correctly with the set circumstances else the consumer news article (or Consumer ;-) gets the wrong message across. I can only speculate because the info is not there, but my engineering conclusion is the percentage is most likely the percentage increase in leakage of heat.

There are so many factors, like timber windows are great insulators for a few years but then they are the worst. PVC is great insulators for ever. Aluminium is okay but with huge variation depending on the quality of (or lack of) the thermal break. Same goes for every other component of the house. At the moment for us the worst is the front door. It is solid timber (builder's default is metal...) and I thought it would be ideal, but we took a T&G door and now find the door frame rubber seal does not seal on the grooves of the timber door so we get a draft. Now to design a solution for that...

A major source of heat escape, after other insulation is done, is the gap where gib joins between the wall and ceiling. The gap that is hidden behind architraves and does not have insulation either. Our house plans actually show there is no insulation where from the wall to the eves, which is a band of about 10cm all around the top of the exterior walls. This is similar to Richms T&G ceiling, cold air leaking through the gap.

If you really want to save money, get a heat pump hot water cylinder. But you need to shop around for a good quality one that is not a rip-off price. Then during summer you route the exhaust air into the house and also get cooling. And if only they would design a heat pump oven.