Couple of areas in my house I need to repair because of condensation and bubbling paint - one shown in image here:

https://live.staticflickr.com/65535/50691875426_9fa7f070fe_b.jpg

Had to hack off some of the multi finish on this one, what would be the best way to repair? The paper layer has also come off the plasterboard so I can't reskim the current surface.

Would it be blue grit and then multi finish?

Do I need to use Wickes Bonding Agent?
https://www.wickes.co.uk/Wickes-Ready-Mixed-Plaster-Bonding-Agent---3kg/p/221807

Or a bonding coat instead of blue grit?
https://www.wickes.co.uk/British-Gypsum-Thistle-Bonding-Coat-Plaster---25kg/p/220055

Could I use easi filler instead of multi finish?

I thought it was a leak but there are no pipes in that area.

Had a damp surveyor in last week and showed him the problem areas where paint was bubbling, he peeled off the bubbling paint and revealed lots of patches of white filler, he said this was the problem. There is condensation due to cold spots as there are no wall rads, only underfloor heating on ground floor, which is never fitted too close to the walls. Lots of filler was used on the plaster, and probably not left too long before being painted. He said he sees it all the time and sometimes around sockets or switches where there's metal backboxes too.

He said it's a straightforward solution to scrape off loose paint, sand down the walls to get rid of all filler, mist coat and repaint.

However just wondering how best to fix this particular wall as it was so bad the plaster had blown and I had to hack it off.

Re: your comments about easi fill, isn't a bonding agent needed first as the paper layer has come off the plasterboard?