I "accidently" (ie randomly first off the shelf in Homebase) bought a pack of these Harris Finesse brushes:
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must say they are the best brush I've every had for cutting in emulsion.They don't hold much paint but they bristles are formed into a razor sharp edge and never seem to splay despite the fact my idea of brush cleaning is use a hose.
I used to use masking tape to get a decent line betwixt wall and ceiling (with all the problems of risk of pulling paint off) but I can get a quite passable line with these and my hands are not that steady.
Cheers Tim