How do I change a two-way light switch to a single?

I live in the UK and have just redecorated a bedroom in the house and discovered that the light switch - which has never worked since I moved in 6 weeks ago - used to be a two way switch. The previous tenant has (presumably, because I can't see it) cut the wire to the second switch, blocked it and cemented it into the wall.

Now, I have just one two-way switch which doesn't work. Can I change this to a single switch and if so, how? When I took the plate off the wall to look at the wires, there are two lots of red, blue and yellow wires, plus one large green wire (earth) wired to the metal casing. I desperately want to get the light working and while I'm sure it's easy enough, I want to make sure I get it right. Can anyone help please?