So an update and final decision! Also thanks to those I have messaged here privately for their opinions because they had experience of some of the optics that I was considering:
HenRun (in particular)
I hope I didn't forget anybody!
I have now investigated the options and had extended trials of the two best that I could get my hands on within budget and which appeared to match my requirements. These were the Oberwerk 12x50 SE porro and the Meopta Meostar B1+ 12x50 HD roof.
Just to reiterate what my need is: something that I can use with glasses that really pulls in distant objects (usually birds) and shows their detail clearly better than the Noctivid 8x, which is my default go-to binocular. It will have to be good to keep up with the Noctivid let alone beat it. I also want it to have the absolute minimum CA because I will often be looking at things that generate it e.g. the moon or solitary birds against the sky. This was the main reason I chose the Oberwerk and Meopta because they both have a reputation for excellent CA performance, unlike many 12x50s including perhaps the otherwise excellent Swaro EL in my brief experience and the opinion of some others here. I also want good glare performance in tricky lighting situations e.g. against the sun or under a bright overcast sky.
Originally I did not think I would need a fast focuser because I assumed the objects of interest would not be moving fast towards or away from me, and so I took a slight risk and ordered the Oberwerk 12x50 SE from oberwerk.eu only on the basis of the reports of others. This turned out to be optically very good: bright, very little CA, a neutral colour balance and sharp almost to the edge (though not a very wide FOV it has to be said). Perhaps not quite as sharp or contrasty as the very best but close and with that 'natural transparency' that I believe only comes with porro or AK prisms. However, I soon realised that following birds in flight was quite possible with a 12x50 but difficult with the stiff and slow focuser on the Oberwerk. Unfortunately, only 3 months after buying it, the focuser on the Obie has also developed flat spots (sometimes slipping altogether) and the diopter seems to be going out of adjustment constantly, and so I am now in the process of returning it for a refund because it doesn't seem like it will last very long mechanically. This is a shame as it presents a lovely image, especially at the price. I'm sure a good example will satisfy many people.
So then I bought the Meostar on sale or return. Comparing colour to the Noctivid 8x (my reference for accuracy) I see a very similar balance with plenty of red present but perhaps with a bit less blue/violet and a very slight amber tinge. I'm not expecting anything to compete with the Noctivid though - especially in a 12x - so that is fine. Anyway, if I had to choose I would prefer 'warmer than neutral' to the alternative of 'cooler than neutral' which is how ELs and NLs always leave me feeling. The EL 12x50 is probably the only real competitor that can be comfortably used with glasses in terms of image quality and with better eye relief, but as several people here have said it may also be less sharp in the centre and almost certainly shows more CA and glare, so that puts me off especially at the much higher price. If I was doing more serious astronomy then I would look at the EL option more closely though because of the wide and flat field, but for me it's secondary to nature observation and birdwatching where my attention is usually right in the centre. Stars and planets look great in the Meostar actually, we had some very dark (and cold) nights recently and I saw both the Andromeda galaxy and Saturn's largest moon Titan for the first time. Apparently the latter is not usually expected to be feasible with a 12x, but there it was.
Current honest thoughts after trying the Meostar in many different conditions for a while are: they are very sharp indeed in the centre for a 12x and with almost no CA even in challenging conditions, the focuser is well-geared and smooth, and they handle potentially glare-inducing light very well. They also feel very solidly built. Colour balance is very nice if a tiny bit warm (slight amber/gold tint) and perhaps lacking some blue. Despite the sharpness and contrast, the view is relaxing and subtle in a way that is difficult to describe. If you look right at the edges of the FOV you start to see distortion as well as focus blurring, but for a 12x mostly being used for long range nature observation and bird ID that is not really an issue for me at all as I am concentrating in the centre 70% which is all sharp. For astronomers who want a completely flat field it might be more of an issue. They are not the brightest optic at dusk, but not far off the NV. The ER could be better with glasses but I have tried my own hack based upon HenRun's excellent idea and this gives 1-1.5mm extra over the supplied eye cups, which is enough and in fact even lets me get too close with my smallest glasses so that I can start getting some kidney bean/blackouts if I push against the glasses too hard, so the flexibility is there if necessary.
So I have decided to keep the Meopta Meostar B1+ 12x50 HD! The combination of excellent image qualities that suit my preferences, good focuser and very solid build backed with a 30 year warranty are too good to give up, even with the marginal eye relief using glasses and the less than top level brightness. I strongly recommend it to others who have similar needs to my own.
