Gavin, just wondering... while you had the 160 out, did you try the 55 or 41 with the GSO R-C reducer instead of the AP .75x reducer?
By any chance, do you know what humidity was on your observing night in London? I have found that higher levels of moisture combined with smoke, dust or pollutants, exacerbates transparency problems.
A little off topic, but maybe of interest, here in the central valley of California, during the winter we get Thule Fog, very dense fog causing visibility to drop sometimes to 15-20 feet... no kidding. There was a news story recently about a scientific study that was conducted over a period of years as to why this region suffers this extremely dangerous weather anomaly. As it turns out, nobody remembers Thule fog here before about 1935. And, more surprising, since the mid-1980s, the serious fog events have been decreasing. In 1975, two-way catalytic converters (CCs) were installed on new cars. Those first generation CCs combined oxygen with carbon monoxide and unburned hydrocarbons to produce carbon dioxide and water. But in 1981, 3-way CCs were installed which also reduce oxides of nitrogen. It seems that the nitrogen oxides combine with water molecules to cause our fog, especially during low pressure atmospheric events which allow pollutant build up in the valley atmosphere. Ever since about 1985, the severe fog events have been on the decrease.
The point is that there are lots of factors that can cause transparency problems for local observers, with or without NV. And we don't always know what causes them. I have always felt that our valley has terrible transparency much of the winter because of fog. Now, at 71, I learn that pollution, mixed with our irrigated, fertile valley land creates much of that problem. So when an observer has a bad night because of transparency, I immediately wonder how high the humidity level was... so long as you don't have any forest fires or ag activity like almond sweeper, harvesters or tilling going on.
The other thing I wanted to mention is that subjective preferences also play a role in what equipment we use. Some like a lot of space around an observed subject for context; others want it bigger and in their face. Just like some of us prefer wider pass-band filters to see stars with our nebulae, while others prefer narrow filters to show a nebulae with distinct edges and bright enough to make it appear as a solid mass. Neither is better than the other; they are just different. So it is with aperture and the speed of optics. We all have our preferences, usually born of experience.
Edggie's point about dark sites is very important. Going from high levels of LP to a dark site is extremely beneficial to NV. At a dark site, there is no need for long pass filters. And at a dark site, I can use a wider pass band filter to see more stars with nebulae. It's just better, significantly better.