most likely the headgasket is gone m8

but you could as said have a leak from somewhere, on one of my rst's I had a similar problem, and the only way I found the leak by accident.

I started it on a cold night and as I drove down the road I could see steam coming from under the bonnet, so I pulled into a petrol station (under the lights) opened the bonnet and looked for where the steam was coming from.

The water return pipe (metal pipe on the turbo) has a hairline crack in it where the bend was and was spraying a fine jet of water onto the turbo housing and burning it off, so if it hadn't been cold would never have found it.

When I took the pipe off there was no sign of it being cracked either, until I pulled it apart very slightly.