Senni,
There is something wrong here and the cause could be one of many factors or a combination of all of them.
INSULATION:
"Sagging" quilt can require insulation to be wired in place.
This can foster "packing" as an alternative - stuffed in, it can't "sag".
But tightly "packing" quilted insulation can seriously reduce its effectiveness.
Vapour Checks are needed to prevent a build up of moisture affecting the insulation.
Insulation has to be actually installed to work - I've seen instances of it being left out completely!
Check the attic to ensure that its been insulated - you'd be surprised what people forget in the rush to complete a job.
Watch out for cold bridges through bunching, sagging, omission or compression and check the attic is adequately vented in accordance with TGD F of the Building Regulations.
FENESTRATION:
7 years ago the quality of window construction, frame detailing, unit sealing wasn't where its at now.
Some window units from 2002 were metal framed affairs and if the frames aren't thermally broken they act as negative radiators.
A preponderance of windows on the North Elevation(s) - or a total lack of windows on the South Elevation(s) - can combine with poor quality units to spew heat from the house.
ORIENTATION/EXPOSURE/VENTING:
Your "front" is to the east - I presume this means a side elevation with few of no windows to the south implying little thermal gain during midday and afternoon - cool summer mornings may contribute little.
Recessed downlighters in any ceiling below a roof can cause significant down draughts when they're not in use - the more, the colder.
Vents on fours sides of a detached house means cross-flows occur all the time, whether or not there is a chimney.
Doors front and rear that open directly as opposed to via a lobby, utility or porch can force air changes through all the vents.
All of this is exacerbated on an elevated or exposed site without topographical cover [hills], vegetation [trees] or urban form [nearby buildings] to provide a decent level of screening.
If the slope of the ground is also against you, i.e. your house is on the north face of a hill, this could be significant.
HEATING SYSTEM:
Unless your gas heater has a balanced flue unit you'll need a separate permanent vent to supply it with air = massive air leak.
Consider replacing this fossil fuel unit with something which has a reduced carbon footprint and if the budget stretches consider augmenting it with ambient energy source heating systems - wind, solar, geothermal.
SUMMARY:
Lots to assess and empirically investigate there.
Obviously the heating and venting and sealing needs to be assessed holistically
Thermal imaging will only assess heat loss through materials, components or composite construction.
You'll need a blower test to check the house properly for infiltration losses and a taper/smoke test to locate the leaks.
You may need to carry out some limited opening up to check the existence of and the quality of the timber frame insulation throughout the house.
Taking off plug covers may prove the existence of vapour check as well as insulation and you should pull some out to see it isn't damp through or wet on the back.
You'll end up being faced with the big decision - whether or not to completely seal the house, remove the gas fire, install a different heating system with an MVHR installation.
If you retrofit this, remember to preserve the Fire Resistance [FR] of the First Floor and elsewhere using fire-collars and other appropriate measures.
Garages and Attic Conversions/Stairs require special FR and escape measures and you may need to put fire hoods on recessed downlightersl.
Take competent professional advice whatever you do, since we can only offer general advices here on AAM.
HTH
ONQ
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