Ian will be the best person to advise you on the WTR I was under the same impression that you were.
You will have to apply for your medicals and police checks again as I think they are only valid for three months. Your police checks will be done in the UK again, I think NZIS automatically do a NZ police check for the time you have spent here. Section 18a is a condition attached to some PR permits that have been awarded due to a job offer. It states that you must stay with that particular employer for three months. Once you have fufilled that requirement the condition is then removed and, if you want too, you can change employer.