OK so you are on the Northern end of the Isle of Lewis. That means predictions based on a postcode will cover a large area and therefore be much less accurate.
The actual terrain and obstacles from an antenna location will matter. But with what we have all I can do is give more generalised advice that specific.
That postcode gives two possible transmitters:
Eitshal - the 'main transmitter' on the Island, a 6 multiplex site - 37km (23 miles) away to the South-SouthWest.
Wolfbane suggests 37/40 dBuV/m depending on which mutiplex frequency at 10 metres above the ground (2 storey house rooftop), and that there is
not line of sight to the transmitter, so signals will be diffracted.
Ness of Lewis - a relay 3 multiplex site - 3km (2 miles) to the East-SouthEast.
Wolfbane suggests 54 dBuV/m and clear line of sight at 10 m and less if the aerial is lower (e.g. a single storey).
Freeview predictions for interference from other transmitters are excellent from both the above transmitters.
Eitshal has "120 channels" whereas Ness Of Lewis is "45 channels".
Other postcodes for "nearby" post offices give similar results, although many suggest Eitshal as the likely transmitter when Ness of Lewis is probably a stronger and easier to receive signal?
You'll need to do your own runs for your actual address house number/name and postcode in Freeview, and the cruder postcode only for Wolfbane.
If you don't have a HD TV set the 45 from Ness of Lewis will be fewer. Check what frequencies are being tuned in and strored?
Check signal strength and quality readings for each one (list them here)?
Check which direction your aerial points: to Eitshal or to Ness of Lewis.
I need the exact aerial and amp details to be able to advise on them and whether the issue is one of 'wrong transmitter' or poor choice of kit or something else.
Long cables attenuate signals depending on cable type/spec and the frequency concerned:
Coaxial Cable Specifications WF100 CT100 WF65 RG6 gives numbers.
Cable losses can be mitigated by using 'launch amplifiers', provided a good signal is received by the antenna (aerial). These can be remote (coax cable) powered if mains isn't available where the antenna is located.
TV and PVR tuners need around 45 to 65 dB signal in to work well (too much is as bad as too little). Aerials may have 10 dB gain and roof aerials 10dB loss so you can see Eitshal is probably not ideal (too weak inside the loft) but Ness of Lewis might be OK? Add a 20 dB amp, though, and then it
might be too much and cause overload?