Advice please! Earth and dimmer

I want to change my broken plastic dimmer switch over to a metal one. Ive taken off the existing plastic plate and inside the plastic box there are 3 wires. 2 of which go into the plate. The red goes into the squiggle with the arrow, and black goes into L2. The other wire is an earth (I assume, as its green and yellow) that is wrapped around a screw in the top left corner of the plastic box.
The new plate I have got is a 1 gang 2 way metal dimmer. (I couldnt find a metal 1 gang 1 way one). It has 3 holes for wires instead of 2 and also a separate screw for the earth. So I was going to put the wires into the same bits in the back, ignoring the L1. (Theres no L1 on the broken plastic plate). Is this correct? Im sure it is after reading the other posts but Im just looking for reassurance! Can I undo the screw in the top corner of the box, pull the wire away and wrap it around the screw on the plate instead? Or will I end up giving myself a nasty shock?! Am I supposed to get another piece of earth wire?
Please reply in the most basic of terms as Im not familiar with electrical terms at all!
Many Thanks!

Turn off the power to the lighting circuit, and then use a tester between of the wires to make sure they are all dead. It is even safer to turn off the main switch to the house. A neon screwdriver is not an adequate tester.

Yes, the G&Y earth wire can be taken out of the screw. Since the back box is plastic it does not need to be earthed, the screw was just somewhere to park the wire. If the earth wire has been cut too short then you will have to get a new bit of G&Y sheathed to link from the screw to the switch.

p.s. It should not just be a screw, there should be a little metal terminal to park the earth in. If not, and the earth wire is not long enough, buy a small connector block "choc-block" at your hardware shop and cut one segment off it. This will give you a single metal connector with plastic insulation round it.

Thanks JohnD. You'll be pleased to know its done and I didnt injure myself! Laura