Thanks for the help, will run the lot through some filter paper and see if that helps. I just ran another batch, and getting some tombstoning, so probably will upgrade to a PID controller at some point soon.
RE the thermocouples, the one above the board is there so that i can be sure that the whole board has reached thermal equilibrium before this sensor hits 230. Regarding the other one, I start from cold with the board in place, so having a sensor that lets me know how long until the vapour cloud forms is useful as i am operating this manually.