WAGO Inline Splicing Connector (Pack of 60), 4 mm², 2-Conductor, 221-2411

At 30p each, this appears to be by far the cheapest way to pick up these in-line connectors – theyre nearly twice the price per unit when buying the smaller boxes.
The other format 2-connector version were recently going for 15p a unit in packs of 100, but these in-line ones seem to be a lot more popular.
Connection of solid, stranded and stranded conductors from 0.2 to 4 mm² in a straight through direction
Minimum space requirement due to slim dimensions for confined installation spaces
Tool-free connection and disconnection of conductors thanks to convenient lever technology
Position fixing and multi-pole installation possible with adapter
Only to be used by qualified personnel