Yeah, I thought Yellowhammer was one possibility but didn’t mention it because the OP seemed very sure on them being Shorelark.
The trouble with identifying birds long after the event, which often for inexperienced birders is unavoidable, gaps in our observations at the time tend to not stay as gaps, it seems as nature abhors a vacuum, so our minds have a logical tendency, in the process of assimilating incomplete snippets of information, to want to ‘complete’ the picture even if it means filling the gaps with predictive imagery to make sense of the memory. The danger of that is we can end up with a form of ‘false’ memory of what we saw unless we are aware of the risks of doing so.
What I would say here, however, is that I had a good, long look at both the bird I think was a shore lark and the two black and white birds. There are countless times where I've felt I just haven't had a good enough, long enough or close enough look to make it even worthwhile looking in my bird book because too much guesswork would be involved and any conclusion I arrived at would be meaningless.
With these birds I had a very good look. Good enough to be able to run through my head: size, head/body/tail ratio, colour on the back, colour on the breast, any mottles and so on; and repeat that a few times. I was very lucky in that they came into an opening and stayed there for a while. In fact the two black and white birds would have been there longer were it not for dog walkers coming past. As soon as they flew off I opened my bird book, with the image fresh in my mind. At that point, I also wrote down the details in my book that I use for birds I haven't seen before (which are getting fewer by the week).
The only point I would make regarding identifying birds after the event, is that some people are more reasonable than others and that would apply to bird watchers too. I fully accept that I haven't been through the range of experience that experienced bird watchers have been through, and that quite clearly means there is more chance of someone like me getting it wrong, and I fully understand all of your points about mistaking size and so on from a distance. I have no desire to fill in the gaps because I'm interested to know what I'm seeing as opposed to tick birds off a list. As an example, I saw the back end of a bird fly past in the same valley about a week before. The back end of that bird would suggest an owl but I didn't even bother looking in my bird book or even mention it to anyone because I simply did not see enough of the bird to make it worthwhile.
I'm enjoying the conversation though, so thanks for the replies.