Hi staryeyed, we got the Electromagnet and Slimline Bracket from Toolstation, cost £56.84 total.
Then we got a 12 volt, 1 amp power supply (it's a little transformer with a 2wire lead leading to a jack plug). Around £5
We also used 2 Narrow press to exit switches from Screwfix £15
You need a normal plug socket to plug the transformer into. Preferably one located where your DS couldn't switch it off 
You fit the electromagnet (not too difficult at all, instructions are good).
The switches are wired as follows:
Take the power supply lead and split it into the two wires. One will have a black and white casing, the other plain black. The black/white lead is the live. The plain black is neutral.
Take the lead to the switch that will be nearest the power supply (ie. the one on the same side of the door as the socket). Trail the lead to the switch, to get the right length, then snip the 'live' wire.
Connect the live wire attached to the transformer, to the 'common' terminal on switch 1. Then, connect the other part of the 'live' wire to terminal 'L2'.
Drill a hole in the wall between the switches. Pass the rest of the wire through.
Snip the live wire again, making sure that there is enough length between the two switches. Take the live wire, which is connected in 'L2' of the first switch, and connect it into the 'common' terminal of Switch 2.
Take the remaining snipped end of the live wire, and the neutral wire, and wire them into the terminals on the electromagnet.
What you end up with is a 'serial' circuit. The electricity goes to switch 1, then switch 2, then the electromagnet, which is energised, so the door is locked.
If either switch 1 or 2 are pressed, the circuit is broken, and the electromagnet loses its charge, allowing the door to open.
It is important to know that in a power cut, the door would be unlocked
This is known as 'fail open'
If you wanted it 'fail closed' you would wire into L1 of each switch, which would mean that the default would be unlocked, and in a power cut the door locks. Not good for our children 