Worcester 25si?

I have a new boiler nearly 6 months old. Worcester Bosch 25si.
For the past 6 weeks or so I have been having to fill it up every single day as the pressure is on zero.
There is no evidence of a leak in my home or none of water coming from the overflow. The company who fitted it says the water has to go somewhere and B.Gas has informed me that the radiators are full of gas turning to hydrogen and needs flushing out correctly at the tune of £500. THERE IS STILL NO LEAK INSIDE TO SEE.