Baxi water pressure fault?

My Baxi was installed 18 months ago and was fine until developing a water pressure fault 3/4 months ago. The maker's engineer said there was a small leak which he had cured, but now we having to top up the water pressure daily. It was suggesed that the problem was in the central heating piping which had been flushed out during the Baxi's installation, but if thats right why was the leak not present from the start?