Handbrake should bite on three clicks.
If it does not, undo press-stud on said cover, there is a 13mm nut to adjust the handbrake, tighten until handbrake bites on three clicks. Ensure the rear wheels spin freely when in the off position. If this does not do the job, examine state of rear brake pads, if below spec, replace and adjust handbrake as described.
If all that fails it will be down to stretched cables. There are two on the 146/6, one for each side, fairly easy job but has to be tackled from underneath the car. Slacken adjustment off until cable can be disconnected at caliper end, then undo at the various attatchment points until it can be removed.
Hope this helps.