Damaged steel wire armoured cable
It would be best to cut out the damaged section and replace it with a suitable gel-filled junction box (or two junction boxes and a new bit, if there isn't any slack). 

https://www.tlc-direct.co.uk/Products/WKSH506W.html
or
https://www.tlc-direct.co.uk/Products/SWJK1.html

may be suitable. 

Your cable size is 3 x 6mm2 copper.