CrossRoads:
Altera an353.pdf
just noticed the attachment in your post
thanks for that 
jackrae:
You also need a bit more integral action as evident by the "controlled" deviation at the high temperature. Integral action is designed to reduce off-set error based upon how long the error exists. It is evident that you are still applying heat even with the deviation existing.
yup I have upped the Ki value and it has improved
also using the same values thoughout now
simplifies the code - always a bonus
more pretty curves later
cheers
Mike