Slow emptying cloakroom sink

[ToKen]

The sink in my downstairs cloakroom seems to take ages to empty. Underneath is fitted an anti siphon bottle trap. I unscrewed the bottle in the summer thinking I may have some sort of blockage but it was clear. The waste continues through the wall behind, into the garage where it joins the soil pipe about 6 feet away.
It is so slow that the basin fills up, even without the plug in, and takes about 5 mins to drain away. You can't actually here or see it drain and we are only talking about a tiny handwashing basin.
Any one help at all?

 

[oilman]

If it's not the trap blocked, it must be blocked further on. You need a rodding spring. Available from plumbers merchants, Wickes, ironmongers etc.

 

[breezer]

sink and plughole unblocker stuff? from shop of your choice