Headset not screwing onto forks correctly

Like R-d-C says really - as it's alloy go easy on the HTFUing ;-)

Oh and TPI is threads per inch - bearing in mind an inch is 25.4 mm you're looking at a little bit above or below 1mm between 'peaks' on the thread depending on your threading type

TPI is indeed threads per inch, I don't understand the second part of this sentence.
1" forks relates to the diameter of the steerer not to the TPI