Mac hostname defaulting to unknown-xx-xx-xx-xx-xx-x in Terminal

I'm trying to fix this one issue with command line that keeps coming back.

I've modified my prompt in the .bash_profile by adding

#FOR CMD LINE GIT AND COLOR
RED="\[\033[0;31m\]"
YELLOW="\[\033[0;33m\]"
GREEN="\[\033[0;32m\]"
NO_COLOR="\[\033[0m\]"

parse_git_branch() {
    git branch 2> /dev/null | sed -e '/^[^*]/d' -e 's/* \(.*\)/ (\1)/'
}

export PS1="$GREEN\u@\h$RED \W$YELLOW\$(parse_git_branch)$NO_COLOR$ "

I wanted the current git branch to show up on the prompt like in the treehouse vm, which is where this started!

The exported PS1 outputs "user@hostname directory current_git_branch $"

My hostname on start up defaults to 'unknown-xx-xx-xx-xx-xx-x', where the x's represent an assortment of numbers and letters. It looks like

Tom@unknown-xx-xx-xx-xx-xx-xx ~$ 

I can change the 'unknown' section by running

sudo hostname Tommbp

which does change it to a glorious

Tom@Tommbp ~$ 

Sadly, following a reboot/randomly, the hostname decides to change back to unknown, which is really annoying and incredibly sneaky.

I've tried changing the Computer Name in System Preferences > Sharing

and also the local hostname at System Preferences > Sharing > Edit (under Computer Name) but this hasn't helped. I can tell you that if I change my hostname via command line and then change my local hostname via system preferences, the cmd prompt returns to 'unknown'

I think it's the local hostname that should appear instead of 'unknown' and it's having a hard time picking up the value I've given in system preferences?

Any help would be much appreciated!