New member in the UK,VW Touran tdi coolant level is going down out of sight over day

[exmilade]

Hi there, posting to introduce myself.

I am a DIY mechanic, although having lost my job in the last year (thanks UK for ditching the Harrier jump jet) I am being asked to do a lot of car repairs these days.

My wife's car is a VW Touran tdi 2005, my mate has a tdi VW Polo (year?) and his wife has a 2006 Audi A6 tdi, so I have a lot of turbo diesels to look after as I drive a Land Rover Defender 300tdi.

My issue at the moment is a coolant issue with the Touran. It's coolant level is going down and out of sight over a day's use. The next morning when the engine is cold, the coolant is still no where to be seen and opening the expansion cap results in a hiss and the coolant level comes back UP into the expansion tank to the min level. I am suspecting a head gasket blown. Any ideas from anyone reading this would be appreciated.

Thanks for welcoming a newcomer.
All the best
Adrian