The amber LED should operate in the following manner:

Amber LED	Power Adapter Not Connected	Power Adapter Connected
Off	Fully Charged	n/a
Slow Flash	Battery Low	Charging
Fast Flash	Battery Very Low	n/a
On	n/a	Fully Charged

There is a peculiarity of the radio; if the switch at the rear is turned to OFF it will still charge, but none of the LEDs will operate This could be why you don't see the amber LED flash sometimes.

If the radio is turned, on and the amber LED does not operate as described in the table, then there could be a fault with the charger. Before you resort to replacing the unit, check that the little springs under the mains adapter block have not become deformed. Sometimes these can bend too far down and no longer contact the adapter block pins properly. To do this, remove the mains adapter block by pressing the recessed tab with a pen and gently bend up the pins as shown in the attached image. If the adapter will not go back on easily after this bend the pins back down a little until it fits back snugly. This should bring the charger back to life.