using a piece of wood is not self defence im afraid,self defence is using enough force to stop the asailant,using a lump of of wood is way over the top of guidlines laid down by the law,a one moment you say you were attacked by a gang of youths,then you say a man appeared from a garden and you thought he was with the gang,what was the man saying and doing to make you hit him with the wood,i do not think you will be able to use any defence of self defence,and depending on his injuries,you could well go to prison for this.