Double Glazing Lock problem?

We have just bought a 30 year old house whose double glazing was replaced in 2002, part of which was the installation of a patio door. The interior handle of this door is very wobbly and feels unsafe. The original 10year guarrantee was not automatically transferable to us as the new owners and the double glazing company want £115 to check the whole installation, to charge us for any remedial work needed and only then are they willing to transfer (for 2 years) the remainder of the warranty to us. Not a good deal which I have refused.

The problem lock has no external grub or other adjusting screws that I can see and was installed by Weatherseal. Any ideas anyone please?