Thanks for all your replies and opinions.  It is great to hear what others think.  
The think with my problem is that this business was not there before us.  They opened good 2 years after we had moved in.  Until then our flat was a place of calm and rest after work. 
Unfortunately, the problem is still ongoing and even after speaking to them and asking them nicely if they could stop the noisiest work before 5pm was completely ignored. 
Actually, up until January 2020 they were also working every Saturday until 9pm. 
I am not asking them to stop working.  I understand how we all need to make money to be able to look after our families. All I would like them to do is to be more considered and try to finish their work before more reasonable hours. But my plea falls on deaf ears.
 

Normal noise restrictions still apply - depending on which borough you live in check what hours are acceptable for noise. Try first asking them to keep to those times, and if that doesn't work you can contact the council (in Wandsworth they usually respond very quickly to reports of noise after 6pm or on the weekends).

The government advice is crystal clear.
Some shops have been designated as essential, and they can remain open to the public. All other shops must close.

Addition advice says that you should work from home if you can. It does not say all other businesses should shut!

Presumably the business was there when you moved into your flat? I’m not sure it is very fair for you to try and now dictate the hours they work because you are at home more. They have a business to keep going at this very precarious time, and your demands could drive them into bankruptcy and cost people their jobs.

I think they should make repairs for you and install noise insulation at their own expense.

They are allowed to work as long as they are not open to the public. I know it must be hard for you, now your home. But many people with a trade need to cary on to earn an income. I have looked into this a lot. And its allowed. Maybe have a chat with them ask if they would mind starting at 9 instead of 8, and finishing at 4pm.

Thanks for your replies. Government's advice is not quite that clear about who can work and who should't.  I am going to try to put up with it for as long as I possibly can.  Eventually, I might need to approach the workers myself and try to negotiate some more reasonable working hours.  These currently stand at 8am to 8pm Monday to Friday. 

But if you are closed and just working there then I think that is OK? If you can't work at home (because you need your tools, furniture) etc. then I would think that it is OK to go to your place of work.

It wouldn't be classified as an essential service.

Is the shop open to the public? If not and they are just continuing to work on furniture for when they re-open or they are selling online then they are not doing anything wrong. Everyone has financial concerns right now and I imagine they are trying to keep their business going which is understandable.

Perhaps call them and try to talk about how the banging is intrusive to see if a compromise can be reached?

I live above a furniture shop, which is still working its full time hours.  Is a such a business considered as essential?  Hammering and stapling  for hours until 8pm.  Extremely intrusive and disruptive while my family and I are trying to isolate.  What shall I do?  Any advice?  Thank you.