I have a 2018 octavia and the drivers door speaker randomly stops working and starts working if I bang on the inside of the door. Has anyone else had this issue and managed to fix it ? 

Likely faulty driver unit. Could be loose connection / damaged wiring -  but the plugs are normally firm fit onto speaker terminal. 

 

They're cheap to pick up second hand (new mid bass driver) but test it before you put all the door card back together and probably clean off old gasket sealant and apply sone sealant mastic as water tends to drip from the speaker seal to the inner door seal unless they are very well sealed.. 

Mine went earlier this year, also a 2018. Replaced like for like with a new unit from the dealer.