If you need to join a cable, and you do not want to / can not use a junction box or chock block (note that junctionboxes and chock block must be accesible for inspection and retightening) you should use a pair of Ratchet crimpers not the cheapo crimps.

The crimps must be covered by heatshrink sleeving or self amalgamating tape for waterproofing if covering with plaster.

the sizes of the crimps are as follows:

Red from 0.5mm² up to 1.5mm² cable Blue from 1.5mm² up to 2.5mm² cable Yellow from 4.0mm² up to 6.0mm² cable