So, I eventually got around to replacing that headphone jack. I had been putting it off as the issue did not happen very frequently, but just recently it has been switching to headphone mode without a reboot. Also as replacing the headphone jack required removing the motherboard I was somewhat trepidatious about it.
No difference though, it still boots up in headphone mode until I cool it down for a period of time.
I will see if it lasts in speaker mode for longer over next few weeks.
In the meantime I have bought another pro5 advertised on amazon as a UK 32GB version. I will concentrate on re-partitioning and putting 16.04 on it.