OMG an omnidirectional aerial with amplifier built in. Probably the most wrong aerial a builder could possibly choose to install.
Of course as with much SLX/Philex stuff no meaningful technical specifications can be found on their website. But it appears from Screwfix and CPC
(User Manual) that it is claimed "up to 28 dbi" = 26dB ref a dipole, thanks to the "20dB amplifier included".
A dipole has 0dB gain. Omnidirectional aerials require that dipole to be vertical which CP transmissions are not...
NB The UK network of transmitters is designed on the use of directional aerials at 10 m above ground. Directionality ensures that unwanted transmitters are rejected
For the Post Office/Nisa shop Freeview predictions from CP are pretty good - albeit 40 miles (64 km distant) - but only with a
directional aerial.
However there is high ground between the location and transmitter hence signals are diffracted rather than line of sight and only 45 dBuV/m.
Sudbury is 51dBuV/m and line of sight.
Other nearby postcodes (Sailing Club and a Farm) are very similar. (Your bit of Essex is fairly flat, I think).
Now let's do some math:
CP 45dB at aerial +20dB SLX aerial amplifier = 65 dB
10 m of cable plus termination to D-A -3dB = 62 dB
Distribution
Amplifier +10dB = 72 dB
Cable and termination to any TV outlet -2dB = 70dBuV
Naturally if the claimed up to 25/26 dB is accurate add 5 or 6 dB to the above... and Sudbury's multiplexes will be received at 6 dB higher than that.
(NB the DA you use can supply the external dome's 12V power requirement from its aerial input. So there may well not be another power unit anywhere.)
TVs ideally need between 45 and 65 dBuV to work well. Too little they lose signal and breakup occurs.
Too much and overload / inter-modulation of signals within the tuners resulting in distortion occurs.
Often a sign is varying quality on a TV meter with high signal strength.
This may well be your problem.
Signal levels at a receive point will vary a bit with time and that may mean sometimes you have trouble (as the received signal is higher) and others its OK (lower). It's probably not interference from other transmitter sources as you might believe.
You may need to reduce the signal in to the DA to a lower level using an attenuator
Splitters, amps and diplexers - A.T.V. Poles, Brackets, Clamps & Aerials
Probably a better technical solution would be to fit a proper directional aerial.
ATV's choice of TV aerials and WHY we chose them - A.T.V. Poles, Brackets, Clamps & Aerials
These would give a gain of 8 to 10 dB on the 45 available, -3 cable, +10 amp, -2 = 58-60 dBuV.
At least in theory.
Terminators 75 Ohm - A.T.V. Poles, Brackets, Clamps & Aerials shows what a terminator looks like.
To split more ways just use the 7th +17dB high level output to feed passive splitters. (2 way -4dB, 4 way -8db cf the input) - though beware the high level and potential for overload.