My 3 week old baby is in a cosleeper, regardless of what swaddle we use and tightness of the blanket (have also tried having blanket looser) he is able to move up the cosleeper and to the side where there is hard plastic in between the mesh (the side you can pull down as part of the co sleeper design) he then bangs his head against the plastic so much so that his head bleeds. We tried swapping the co sleeper around so the side you pull down was facing away from the bed and he was still able to find the plastic side and bang the other side of his head… not sure what to do, midwife said she has never heard of this happening.

Many babies do wriggle & squirm & can be quite active & will move around the bassinet.
Some babies do not respond well to swaddling/wrapping.
Sometimes it is because they like hands free to touch their face or suck on fingers.

What are you using as a swaddle?
Are you able to try him with his arms out of the swaddle or cease swaddling?
You could try moving your baby to a cot with more space, as well.

I agree with your Midwife - I have never heard of a baby causing head to bleed by wriggling to side of bassinet.
If you are concerned about your baby’s health/well-being, please discuss with your GP or other Health Professional.
Have you seen your local Maternal/Child Health Nurse yet?