Chip off all loose plaster, don't worry if you're left with a big canyon.
Buy some cellulose filler, like this:
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a filling knife, like this:
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some sand paper:
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a sanding block:
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cleaned the crack, mix up some filler (instructions on box) and apply to crack, making sure the filler surface is a little below the finished wall surface. Wait for this to set.
Mix up a second lot of filler, and use it to finish off the crack slightly proud of the wall surface. Leave to set overnight.
That isn't a plaster crack, it's in the brickwork, something below this window has moved in the past or is still moving, has there been any widening of doors or other structural work carried out below it?
Its above some patio doors leading to the conservatory. The external brickwork looks fine. Do you reckon I should hack the plaster right off and look behind then?
That would be your best policy. I like Phillip had doubts whether it was just a crack appearing and since you have confirmed there's patio doors been put in down stairs below it makes you wonder.
With respect, given your limited knowledge it is extremely unlikely you would be able to assess the wall meaningfully.
Hacking back the plaster will simply show you a crack in the brick/block wall. There must be one, otherwise the plaster would not have cracked. But what would that tell you?
House walls crack - that's life. It's all part of the natural settlement process. The real question is whether there is a crack that is growing, or has been caused by recent work.
You initially stated that you'd had a surveyor look at the wall. I took this to mean that it had been inspected and passed. If you are uncertain about his, get a structural engineer in to advise you.
That wouldn't be of much use because we already know that the brickwork is also cracked. What I think has happened is that whoever knocked out for your french doors has not put a lintel in on the inside skin, this would explain why there is no corresponding crack in the outside wall, he may not have put a lintel on the outside skin neither but the patio door frame would go some way to holding this up, either way, you are going to have to check for a lintel above this door...it /should/ be steel but could be concrete or even timber. The safest way to check is to chip the plaster off....I'll do a quick diagram of where you should look - *don't* start banging holes out of it in the middle!
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