The warning sign, manoeuvre braking currently unavailable comes when I turn my car key and then it seems like the battery is down.

Any idea why?

Welcome.

 

Could the battery be down on a 5 year old car.

What engine / gearbox have you and does Stop / Start work OK?

Stop/start is not working and it is a 1.6 diesel engine. The gearbox is an automatic DSG.

The symptoms seem to suggest the battery has become lazy.

Yes, it seems something to do with the battery.

Don't forget if you replace the battery it needs to be coded to the car so that the car knows it has been replaced.

 

Replaced the battery with a new one, and now, not only has the warning sign disappeared but also the start/stop function is working perfectly.

So, thank you, you have identified the problem well.