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Thanks!
I did try various searches in the thread, and I thought I remembered @stevo_com in particular doing this very task, but I couldn't find anything specific.
I've just watched a Charlie DIYte video about the same job and he also recommends the PVA priming.
tbc-
I used gyproc easifill on a particularly beefy crack, probably up to 50mm deep in places. But only because it's what I had to hand. I am certainly one whose advice should be taken with all appropriate "I'm just having a go" caveats. But I built it up in a few layers and it doesn't seem to have crumbled, cracked or come apart in 6 or 7 months.
I like easifill as it seems easy to apply, has a decent drying time (given dryish conditions) and sands well. But it does sink back quite a bit. Especially the ready mixed stuff as it's fairly wet so there's a good bit of moisture/volume lost in the drying out (is my assumption). But, so long as I expect to refill/cover at least once, it does what I want it to from a DIY-er perspective.
stevo_com
There's quite a bit of chat about this very task in the thread. The method a few people have had success with is Bonding plaster to a very shallow depth and then a choice of Multi-finish plaster or TX110 or Easifill. I find TX110 the easiest in terms of drying times and blending into the existing walls. You can use Gardz from Zinsser on top of the filler to stop it 'grinning' when it's painted.
Main problem with this approach is if you only need small quantities of the materials as Bonding plaster comes in big bags although it's not expensive.
Prepping the chase for the plaster usually involves a bit of PVA mixed with water splashed about. Just helps the plaster to set without the dry walls sucking the moisture out and making it crack. Some people don't use PVA to prime for plaster but in this case I've never known it to cause a problem, normally more of an issue if you're skimming walls.