One owner has clocked up over 143,000 miles - and has only needed to change the actuators in all that time.
Try to avoid forcing the doors manually as it COULD stretch the cables and MIGHT cause the slack to jam on the motor drum or over-rev the drive motor. Should the electrics fail, just use sufficient force to get the door moving; no more. This includes garage mechanics!
It is assumed you have already tried a door reset (also in your Owners Manual and the Familiarisation sheet if still with the car)
Most door problems are due to the in-door actuator so, as a first step, change this actuator (you will have to at some time anyway). After 8 years or so they gum up and slow down anyway as the special grease only has a shelf life of five years!
They can sound like they are working OK - but are too slow to hit the short time window in the software.
Be aware that, as soon as the actuator fails, the door computer is put into failsafe mode and will not operate any more electrically - even after a reset you will still get the three bleeps.
After changing this actuator (around £50 on ebay) : if it still doesn't work then:
the two 40amp fuses in the passenger glove box shown here are for the sliding door main motors, one for each door. Make sure they aren't blown and also that they are in the right positions.
If your car won't start or the doors won't work even after a reset, try removing the link HS (or SH) in the BSI for 15 minutes and then replace it. This may also reset some of the preferences but should at least get you going!
Measured at the 40amp fuses in the glovebox fusebox - door operating currents can be up to 25amps (sliding uphill) or as low as 2 amps (sliding downhill)
Mine are around 12-15amps opening uphill on my drive; 2-3amps when closing downhill
If you buy a PP2000 kit off ebay, it will come with the necessary cables to connect to the OBD port in the glove locker fusebox. It may give you a guide to the location of the fault. Although I have PP2000 myself, I've not needed it yet and it tends to be more time consuming (on my lap top anyway) than going through the above steps!
Good luck - the 1007 doors are actually quite simple!