Something I used to use in the landscaping/building trade was a product called milliput... it can be applied to any wet surface, just needs to be reasonably clean.. not cheap but guaranteed it will do the work,, its flexible to so as the joints expand/contract it will expand and contract with it..
Its ace stuff,, but not cheap..
Plumb base have it , and you may be able to get it from somewhere like screw fix..

Its ace stuff,, but not cheap..

Plumb base have it , and you may be able to get it from somewhere like screw fix..
