Or can we just repoint, knock off and replaster with waterproofer added
to the plaster mix?

Is it an old house (how old?) or new?

Victorian terrace

Make sure that whoever re-points uses either lime mortar
(unlikely for most builders as it's a PITA for them) or
1:1:6, which is much more likely and OK for builders -
not straight cement:sand, even "with a plasticiser, mate,
so it's OK" (because it's not OK). Just a small point

Whats the 1:1:6 mix of? Also what would be the implication of using
normal sand/cement mix (with plasticiser)?

Thanks.