What is the error in this sentence? It doesn't sound right.
Hello, Jamest83, and welcome to the Grammar Exchange!
There are two grammatical errors of the same type in that sentence. In both of the "which"-clauses (relative clauses), a singular verb is used; however, since the antecedent of "which" is plural in each case, and since "which" functions as subject in both relative clauses, a plural verb is needed. Below I have highlighted the head noun of the antecedent of "which" in each relative clause and have changed the verb of the relative clause to agree with the number of "which":
"Please see Annex 1 and 2 for case studies on our existing clients within the healthcare sector which shows show how we have successfully implemented recycling procedures which has have reduced overall waste volumes and increased recycling rates."