Esp and fault codes

[polo1.9TDI SPORT]

HI there,

Recently my Esp has been turning itself off after say 10 minutes or so of driving. The problem is that i cannot press the switch to manually turn it back on.

This happened on Monday twice, to and from work.

Tuesday it was completely fine and i did the same journey, same speed, etc etc.

Wednesday it turned off and by itself turned back on.

So i have been to my garage who hooked it up and came back with the following fault codes -

17911 load signal from alternator implassible signal intermittent (P1503)
18265 load signal error from ecu intermittent (P1857)

I had them reset all codes and this morning esp again turned itself off.

The alternator was checked for output and was around 14.09 - 14.13.
Garage said this was ok to get through winter and may be a potential cause of esp warnings due to the voltage dropping.

Now does anyone have any ideas as to the fault codes- or had this problem?

Any help greatly appreciated!!!!