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At the beginning of the year we had issues with condensation in the semi-detached 1920's house we have. We took actions to address it. However in one corner of our living room(external wall) I've noticed the plaster is starting to bubble and salty deposits forming on it. It is on the lower part of the wall . Does this mean the condensation problem is back? Would removing the plaster in this corner and replacing it with new help the situation?. Any advice appreciated.

Remove and replace not likely to cure it, you need to establish what and where the problem is, could be condensation could be water ingress or a damp proof failure.
You need to find the source of the problem,
what was the orignal problem did it effect this area and how was it treated?
KB

Originally the problem was condensation which we treated by installing a vent in the living room, bought a de-humidifier etc. The corner of the room with the current problem, at the beginning of the year we used sand paper to remove faulty plaster and then used Smooth Over product to even out the plaster before painting it. The wall feels cold but doesn't feel damp. The damage starts above the skirting board and goes about a metre up the wall. The plaster is bubbling and salty deposits have formed but there are no black marks on the wall.

Sounds very much like rising damp this is likely to be caused by a DP (damp proof) failure, Really when you originally did the plasterwork it should have been striped back to wall and start anew. But if the problem is still there, it would not of cured it, cure the problem first.
You can buy DIY injection kits to solve DP problems. Some advertised on this site, can't recommend this as not really used them.
It's not really my field but it will be worth getting an expert in to confirm my thoughts, you can buy the moisture metres to test for yourself if you wish.
Check this link out and browes through the relative topics, it could help you determining your next move and also offer a little more insight.
https://www.diydoctor.org.uk/alphasearch/D.htm

I HAVE HAD 2 DIFFERENT QUOTES FROM PLASTERERS ON HOW BEST TO RE-DECORATE MY DAMAGED PLASTER WORK FROM CONDENSATION.
THE FIRST RECOMMENDED TO HACK OFF THE DAMAGED PLASTER WORK AREA AND PUT IN FOIL BACKED PLASTERBOARD. SAYING THIS WOULD PREVENT THE EXTERNAL SURFACE WALL GETTING SO COLD AGAIN AND DEDUCES THE CHANCE OF THE CONDENSATION COMING BACK AGAIN.
THE SECOND PLASTERER SAID THIS WAS NOT NEEDED. HE RECOMMENDED JUST HACKING OFF THE OLD PLASTER WORK AND REPLACING IT WITH NEW.
IS FOIL BACKED PLASTER ANY GOOD? WOULD IT HELP IN THE CORNER OF AN EXTERNAL WALL WHICH WAS PREVIOUSLY PRONE TO CONDENSATION. ANY ADVICE?

We'll it's a moisture board and will help, are you happy that it is condensation, with a cold spot hitting warm air?

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Hi mate. Don't use foil back board. Using foil board is just covering up the problems and salt would eat at the board. Hack off sand and cement strong mix say 4-1 and use a top branded salt ihibitor like hilka. Prob not dpc failure , more like salt damage. When you come to paint use an anti fungal paint, 1st coat the walls with paint and leave for 48 hours then 2nd and 3rd coast if you need to. Don't use foil and don't get a damp proffer in. 30 years ago I started my plastering training with my dad (a damp proofer) save your money and insist on sand and cement, backing pLaster is no good. Good luck

[quote="kbrownie"]We'll it's a moisture board and will help, are you happy that it is condensation, with a cold spot hitting warm air?[/quote


Foil backed boards are insulation boards not moisture boards. Moisture boards are green used only in wet rooms and likly wet arears ie splash backs, blue board is for sound proofing, pink board is for fire grading and years ago they had a yellow board which was expensive to buy and was used for industrial sound proofing.