Checking my sister's car over for MOT today and have a problem.

It is 2004 Ford Fiesta 1.4tdci

When I turn the headlamps on it is stuck on main beam, I can't get it to go back to dip. Well it does occasionally after flicking the dip/main beam stalk about 50 times, but as soon as you put in on main beam again(it has no problem going from dip to main beam) it is very difficult to get it back to dip. The switch moves fine, but the beam does not change.

I know the main beam relay is not stuck as when the lights are off and I use the flasher the main beam flashes OK and I feel the relay click if I hold my finger on it. 

On old cars it was obvious how the switch worked as you pushed it away from you for main beam and moved it towards you for main beam so it was a simple two way switch. But these modern ones move in the same direction to flick from one to the other. So how does that work, is there some kind of ratchet thing inside the switch or each time you flick it, the switch puts the same contacts together which sends a signal elsewhere in the car to swap the beam. 

I don't mind buying another switch if it seems pretty sure it's a problem inside the switch but I do nit know if that is how it works.

Any helpful comments gratefully received.

(car also needs one tyre- my sister actually drove it here with the rear left tyre totally flat without realising it - air comes out around the valve stem but of course the tyre is now damaged. Also needs front discs and pads - the discs look horrendous and brake very unevenly - I can easily sort these other things)

I have now managed to find a wiring diagram and this makes me think the fault must actually be inside the switch. So I have ordered a 2nd hand switch on ebay for 6.99. fingers crossed it works (there were one or two cheaper but they did not offer the pickup from local Argos shops which I needed)