2 way lighting problem

When we moved into our present home,(a Leech built early 70s semi) the house had been altered to an open plan layout downstairs which involved removal of the entrance hallway wall,on which the stairway light had been attached.
The switches were moved to another wall.
Finally the problem.The upstairs light should be able to be operated from upstairs and down.It has never worked that way.if the downstairs switch is ON the upstairs light operates but cannot be switched OFF by the upstairs switch. If the downstairs switch is OFF the upstairs light can be turned on and off quite normally by the upstairs switch.
We have put up with this state of affairs for some years but recent alterations to the kitchen exposed some of the wiring.
The downstairs switch has an obviously non-original twin and earth feed. The upstairs switch has what is obviously original wiring of 3 wire and earth.
Any idea what is going on?
Also,a pen type power tester shows 12v on the lighting earth-is this part of the problem and is it normal or possibly dangerous?

Post clear photographs of the conductor cables and terminals for both switches.
Also look here:
//www.diynot.com/wiki/electrics:lighting:two_way_lighting

Ideally you need to use a multimeter to check for voltage, if you place one probe on a known good earth and the other on the suspect earth you should get a better reading (volts AC).
It hasn't been wired as a two way if you only have a twin and earth present at the bottom switch. You will need to replace the cable with 3c and earth to do it properly.

Could just be a bad connection at the switch, or it could be a dog's dinner where they have jointed the cables together badly/ wrongly.