If you have washed them then no you don't need to use nematodes. I tend to add in a bit of grit to make it less appealing to the female when she comes to lay her eggs. good garden centres should sell the nematodes as a 'powder' containing their eggs that you water onto the pots. they have to be at a certain temperature to survive however.

I have a problem with vine weevil and use SBM Life Science Provado 500ml Vine Weevil Killer2. You can only use it on container plants and it's best to use a preventative and treat the plants before the weevil attacks. I am not an expert but it is my contention that nematodes are useful but when they've killed all the weevils or eggs, they will die themselves.

Helps as well if you use a non-peat compost and then put at least an inch if not more of fine grit around the plants . They do not lay their eggs if they cannot reach the soil.

Thank you to all who replied ...much appreciated.