So, I was advised by a herbalist of where I could get a 'cheap' Vitamin D test done - City Hospital, Birmingham, for £28. You can find it online, pay the money, and they send a kit. You then need to prick your finger with a special blade, and drop 4/5 drops on their special sheet. NB, the drop needs to be quite large, but shouldn't incorporate several drops - that may mess up the test somehow (it wasn't quite clear how unfortunately since I had small drops and added a bit of blood here and there to make the size bigger). The website link is: http://www.vitamindtest.org.uk.
You need to do the test on a weekday so that they can get it and return in the week. I guess the blood may get too old and not fresh otherwise to do the test. So, Monday to Wednesday are probably good days, and post as early as you can to have good chance of getting through quickly and for the next day.
I was heartened to get the result back (early August), with just into 'normal' range (51, and normal is 50-200). This is encouraging since a test I had a few months ago was 35, and before that last year sometime 49.
It is hard to know cause and effect, but I have made a lot of effort to get in the Sun when it shines, with a fair amount of skin exposed. I've also kept going with trying to increase oily fish (eg mackerel, salmon, sardines) and greens as well as seeds (sunflower, pumpkin, sesame) in my diet. I had already given up coffee, tea and alcohol round October 2016, in the thinking that that too might be having adverse effects in allowing for uptake of Calcium etc for bone. (I have since gone back to having some of these 3 latter drinks from time to time, but so far not as frequently as when I gave them up last year).
I offer you these thoughts in case helpful. It seems encouraging that the Vitamin D level has gone up to within normal range, if still rather low. It's also worth noting we are now in Summer, so a concern that it will fall again in Winter. Something to watch - perhaps I will have another Vitamin D test in about 3 months to monitor.
Next steps may be to review my exercise some more - eg include more 'weight-bearing' exercise which is meant to be good for bone. This might include jogging; skipping is also considered to be very good for bone. My herbalist also recommended mini-trampolining which has been fashionable, but I can't imagine how I could include that into the space I have so probably not an option.
Feel free to post any thoughts you have in the Comments section.