My apologies at mixing up the Chesterfield/EM directions. However, if you are aligned on Chesterfield then that explains why there's no COM 7 & 8 signals.
I wouldn't worry about picking up Sheffield. It's a relay transmitter, so it is polarised vertically. That means the three legs of your boom will fan out parallel to the ground if the aerial is correctly aligned for the Sheffield transmitter. Emley Moor is a main transmitter. It is horizontally polarised. The orientation of the aerial is the same as you'll see in most adverts for it.
Horizontal and vertical polarisation is a way for two transmitters to operate in the same area with significant interference with each other. With TV transmitters, the relays are there to fill in any coverage gaps from the main transmitter. In the case of Sheffield, the hills on which the city is built form a natural barrier to the signals from EM. The relay fills in where the EM signals don't reach. The relay is polarised differently so that in the fringe areas where the signals do overlap, the two don't interfere with each other. The receiving aerial is polarised to match one or the other transmitter, and so the aerial becomes far less sensitive to the signals from the opposite polarisation.
You'll also notice that the Sheffield relay operates on far lower power. It doesn't need to reach that far. That means that the signals from it will be far weaker by the time they reach your home.
I've only used tri-boom aerials such as your LAB450T on a couple of jobs. They were both over a decade ago when I was just getting in to aerials in a small way and really didn't know too much. I was persuaded by someone I later found out had only marginally-better knowledge than me at that point that these were the best I could get. Certainly reading the blurb and some info online, the idea of being wideband and very high gain sounded ideal.
Who wouldn't want that combination? I came to realise later that there was far more to it than that.
Most TV aerials have a lumpy gain curve. That means they're only 'high gain' for a small portion of the reception range. Your tri-boom follows the same pattern. If you want to see what the gain curve looks like then there a very good website from a company very close to where you live:
aerialsandTV
When we look at the coverage checker you linked (and thanks for that), the main Emley Moor muxes are all high power (174kW) and coincide at frequencies where your tri-boom aerial is relatively efficient. COM7 & 8 are roughly 1/3rd of the power, and at frequencies where the aerial isn't so good. Once your aerial is aligned on EM though, the coverage checker still predicts decent reception for those two muxes.
If anything, the combination of a strong signal from EM where you live (the Field Strength) and the peaky high gain of the tri-boom would make me concerned about over-saturating the tuner. If you also have an amplified splitter, then that could be adding to the problem. Too much signal is just as bad as too little, but people rarely recognise this when it occurs. They usually think in terms of adding amplification, and not about reducing the overall level with some attenuation.
My home is about 10km further away from Winter Hill than yours is from EM. The WH signal here is very strong, even though the transmitter is lower powered (100kW vs 174kW). My aerial is a low gain Log Periodic (see the shallow black curve on the graph from the ATV site), and yet I get 100% Quality and 90% Strength on all the muxes I can receive. In fact, there's enough signal coming off this low gain aerial that I could passively split and run three TVs without an issue. I just wonder if you really needed such a big aerial?
If you're still having problems receiving COM7 & 8 then try tuning in channels 32 and 34 manually. If that's still a problem, try bypassing any amplified splitters and have a think about insering a 0-20dB variable attenuator in line to see what happens when you reduce the signal strength.
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