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I'm not finding plasterers that cheap. In fact it seems hard to get them at all in West London.
I'm also not that convinced by the quality of most of the skims I've seen done in the last 10 years and plaster should really be left a considerable amount of time to dry before you paint it, maybe less with a skim but it's still months until the crystal structure has formed.
With all that taken into account I don't see the advantage of skimming when the solution to the problem is good filling technique and proper priming before painting (Zinsser Gardz).
Poor technique :-)
Though going over with watered down emulsion is the best way to show areas that need attention