Wild guess: late eighties or early nineties? Are you based in Manchester? I think you can still get new Harry Hall branded bikes, although I'm not sure who makes the frames for them - at any rate, they've got a few bare HH frames upstairs at the back. It might be worth popping in and talking to whoever looks after that side of the business.
I worked at Harry Hall's around that period. He gradually phased Reynolds 531 in around 1989/90 and only sold them as touring frames with pannier eyelets. All the race frames went over to Columbus slx. I would say this is a 1981-1986 model as the bottom bracket gear braze-ons are external and Stronglight cranks were not stocked by 1990 either as far as I can remember. Your forks are not originals, 531 originals were embossed HH and usually painted the colour of the frame. Also are the decals modern stick-on ones? (if so it's post 1987), before then they were always wafer thin water applied decals and clear lacquered (much smoother, better, but more time consuming.)
Hope this helps albeit maybe 18 months too late!
I had one of these 531's I bought direct from him. He built it to my leg/forearm measurement, How silly was I to sell it. Also may I add Nick Saunders (off the telly) got into the Guinness Book of Records in the 1980's by riding a Harry Hall 531 around the world in 79 days. It used to be displayed in the shop on Hanging Ditch in Manchester and was pearl white with red decals and a shimano 105 groupset. It came back an absolute wreck!