It turns out it the culprit wasn't T1, which others here had suggested to me was unlikely.

After unsuccessfully trying replacement equivalent Q1 & Q2 transistors, I then decided to reinstall the original transistors that weren't originally working. 

Then mysteriously after putting the same transistors back in my Tr-707 it started working again and no longer was switch off after 30 seconds or so. Its been working for the last 4 weeks without a problem. 

Very strange maybe it was a bad solder joint, or one of the transistors was behaving funky but has now decided to behave..

Oh well, happy days, I have a working 707. Time to make so techno :)