My 11 week old baby moves around the cot a lot, and is frequently banging his head against the side of the cot. This wakes him up every couple of hours. I move him to the other side of the cot, but he can shuffle back and bangs his head within 20mins. How can I get more sleep safely?!

What you baby is doing is not uncommon, particularly when baby is close to rolling.
Most babies are rolling from 4-5 months - some earlier, some later.
I think its time to manage him as if he were rolling.

Ensure the following:
Move from bassinet to cot (which you have)
Cease swaddling (or swaddle suit)- baby needs arms out to roll safely.
Ensure mattress is firm & flat & that there is noting loose or soft in the cot.

Always place baby on back for sleep.
Increase tummy time when awake.

Red Nose does not recommend the use of bumpers, especially if he is learning to roll.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows
Sleep positioners are also not recommended.
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position

What you baby is doing is not uncommon, particularly when baby is close to rolling.
Most babies are rolling from 4-5 months - some earlier, some later.
I think its time to manage him as if he were rolling.

Ensure the following:
Move from bassinet to cot (which you have)
Cease swaddling (or swaddle suit)- baby needs arms out to roll safely.
Ensure mattress is firm & flat & that there is noting loose or soft in the cot.

Always place baby on back for sleep.
Increase tummy time when awake.

Red Nose does not recommend the use of bumpers, especially if he is learning to roll.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows
Sleep positioners are also not recommended.
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position