Blimey Howard, illegal fishing on Belgrave Road, didn't realise the Dour had a tributary up there!
I'm a member of the Bureau of Freelance Photographers and carry the 'blue card' they issue to members which confirms they have taken taken legal counsel and there is no law to stop anyone taking photographs in a public place and no individuals can stop you photographing them in public.
(Just edited out the image of the card from this post as it is copyrighted and being Sunday there's no-one available to say it's OK to use, though I'm sure they wouldn't mine me promoting their activities)
There could however be good reasons to advise you to stop - for your own safety, for example. Or you could be advised not to post as it might prejudice a prosecution - papers and tv often publish pictures or videos to help apprehend a suspect but then those pics aren't used again until after the prosecution.
Howard, I missed the last few posts while I was typing - switch the flash off and put the ISO to max next time!