I have read your statement as saying that the builder returned within the defects liability period and did repair work on the loo but this work did not cure the defect. If this is so and your contract permits you direct contact with the builder (ie not exclusively through the architect) then give the builder 14 days to complete the work and if he does not then without delay employ somebody else and proceed as you say. If you have to go through the architect then address the same letter to the architecr and address the summons to the architect and builder jointly and severally. The architectural profession is a grossly incompetent profession.