Retain the rest of the money until a rep from whatever product has been used has come out and looked at the job and has said it is up to spec and ask for a BBA certificate, just ask your local builders merchants who supplies whatever render it is for the local reps number ask him if they are an approved applicator and to come and have a look at the job, your back is covered then if it's not done to standard and you want to keep the rest of the money, some customers can be really awkward for no reason, but this time you need to be awkward because the job you had done was oughta s**t.