I will obviously need to do some filling, not only for any gaps between the edges of the insert and the hole but also to bring the level of the insert up to the surrounding skim coat. Can anyone recommend a particular type of filler/plaster/whatever for doing this? Ideally, a product recommendation such as Polycell XYZ would be ideal as there appears to be a bewildering array of products out there, many of which I'm sure are duplicates under different names or otherwise equally suitable for the job but a personal recommendation of 'the best' based on past experience would be very helpful.
I shall be painting the walls and so am hoping for a similar surface finish to (painted) plaster so as not to show up afterwards. Also, this is a one-off task so paying a premium for ready-mixed filler is entirely acceptable if it aids in the likelihood of a successful job first time around.