Hello there, will do my best to explain what I'm wanting help with:

I have been having pain around the mid-back lower left teeth, which can be felt when pressing around the jaw and hurts when I run my tongue along the gum line. I was also having pain when eating on that side. Only yesterday I notice 2 small white-ish/transparent paches on the lower gum line which I think has now become one thin but long line (1.3cm I'd guess).

I started using a mouthwash last night, today I have a more medical mouthwash designed for gum disease and gingivitis. I took a paracetomol this morning, the pain has not noticably returned and I no longer feel pain when running my tongue around my gum or when pressing fingers outside the mouth.

I haven't brushed my teeth yet (will do before bed), so I don't know if it's still very painful to do so, but I will still take it easy.

Basically I'd like to know if this sounds like anything serious and would appreciate if someone could tell me the name for what it is so I can read up on it and also find out how to prevent it happening again, once cured.

Thank you

You may have an abscess or some kind of infection, you should have your dentist check it out.

Another day has past and it seems the white patch is getting even thinner and the pain has almost gone completely. Would this mean it's clearing itself up or is still a concern?

That's good news, just keep using your mouthwash since it's the medicated kind. I would use it for at least a week and I hope it gets better.