Steering problem

Steering faults on that car are quite common , this shows you the set up .

http://www.ecutesting.com/vauxhall_meriva_electric_power.html

in brief,how does steering work without fluid/pump unit.how does the power assistance work?

This should explain how it works ,

The Meriva (punto corsa) use a electric motor that is attached to the column using worm and gear set up to drive the main column.
In this set up is a column, motor torque sensor and an ECU. the torque sensor is the biggest failure of the column ( and is the same sensor fitted into the punto). The EPS will and can fail switching off the assistance which can be very unnerving.

can be caused by powersteering check error codes for C1500 or C1532 is power steering sensor problem.

Just replace the vauxhall meriva steering angle sensor. vauxhall meriva torque position sensor!