If you need to join a cable, and you do not want to / can not use a junction box or chock block (note that junction boxes and chock block must be accesible for inspection and retightening) you should use a pair of Ratchet crimpers not the cheapo crimps.
The joint must be covered by heatshrink sleeving or self amalgamating tape. This is to ensure waterproofing if covering with plaster and to comply with the requirement for all live conductors to be double insulated.
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