We did the pressure test for around 1-2 minutes. It didn't leak.

We had it on the ramp, no drips.

Somebody suggested this :

'I had to do a head gasket on a Mondeo a feew years back. It was the pipe exactly above where your leak is coming from. It was a metal pipe about 15mm in diameter. It had rusted through and I remember when I bought the new pipe from Ford the parts guy said these go all the time. It may be that its just when it is full upto pressure that it leaks as steam. Not that noticible on a short journey.
'

What do you guys think it is?