how can I stop a drip on a u bend?

If the U-bend is not broken it can only be that the connections at either end is slightly loose.
This assumes it is a plastic u-bend....
I suggest that you put a basin under the u-bend and remove it to examine the seals - take care because it will be full of water - hence the basin - assuming all is well you can then replace it - tighten by hand only as this should be sufficient to seal the joints...

Robert Welsh
May 2008