You will need to replace them (or get an electrician to replace them).
Replacing sockets is a straightforward job:
- Turn off the power to the receptacle.
- Verify that power is off at the receptacle.
- Unscrew the old socket from the box.
- Make a note of which wires go where.
- Remove the wires from the old socket.
- Wire up the new socket - double checking that live -> live, neutral -> neutral and earth -> earth.
- Screw the new socket onto the box.
- Turn the power back on.
This is still a job a householder can do in the UK. However, if you are uncertain about any of the above steps or you think there might be something more serious wrong with the electrics then get an electrician in.
If the plugs aren't holding in the sockets then the sockets are potentially dangerous.