This solution was largely down to what I had kicking around the garage. You can pick up a car central locking solenoid for about £4 on eBay, the alarm contacts are even cheaper.
The circuit is simple:
A 12v power supply is plugged into a simple 24hr timer set to when you want the door to open (set for shortest interval). This runs to the solenoid, operating as a door latch, with the alarm contact in circuit. The alarm contact should be closed, i.e. on, when the door is closed thus completing the circuit. When the door opens, the circuit also opens as the magnet on the door moves away from the contact.
It is important not to leave the door solenoid powered up as it could burn out. Wire it so it retracts when it powers up. It should NOT be powered up when the door is closed! All it should need is a brief pulse sufficient to retract the shaft and unlatch the door. The circuit needs to be broken as soon as the door opens.
You could adapt this to work with a free-running sliding door (maybe add kitchen drawer runners?).
Cable connectors or soldering kit
Central locking door solenoid
Magnetic alarm contact (normally closed)
24hr mains time switch
Cable ties/cable clips
Check out eBay for cheap parts