Bathroom sink not draining

[Mystique]

I recently had a new bathroom fitted. The toilet, shower and bath tub seem to drain fine, however the basin is not draining at all. Initially, it was draining extremely slowing and I could hear a small trickle going down the drain, however it has stopped draining completely since this morning! At times, I can also here some noises (possibly trapped air?) from the bath tub drain.

The guy who fitted the basin has been around to take a look and he has confirmed that there doesn't appear to be any blokages. Although he can't explain why the sink is not draining! Can anyone advise what the problem could be please?

 

[Robster_1981]

Where is the basin in relation to everything else? Could be that the waste pipe doesn't have sufficient fall.

 

[Mystique]

The basin is located next to the shower and opposite the bath tube. I have a square bathroom, with a toilet and bath tub against one wall and a shower and basin on the opposite wall.

Greatful for any suggestions.