Toilet constantly draining
Right ok, whilst I'm still not an expert by any means... but... I think that it's fairly easy to diagnose problem. 

First I checked out a few youtube videos on the problem, most people suggested the issue was the flush valve/cistern (couple of videos on fixing that here, link 1, link 2. 

I actually bought and replaced the flush valve on my toilets except that wasn't the problem.

Part of what really helped me was trying to take my time to realise why it was still running after the flush. I turned my water pressure lower so it would fill up at a slower rate, then I flushed the toilet again and watched each part until I saw what wasn't working as it was meant to.

The first link above helps to describe the process, but in very simplistic terms for people in a similar situation, when you press the switch it presses a release on the valve to eject all the water in the tank, once it reaches it's 'empty' state (which you can change on your valve) the mechanism will close back up and create a container for the water to fill back up again, this is step one in trying to figure out the issue, it's why a lot of the problems are related to the seal on the valve.

That wasn't my issue, then, as the water is filling, the level rises until it reaches the float gauge (where the water is coming out) and the float is then intended to lift the lever on the gauge to close it when it reaches it's fill capacity (again, you can change this)

This was my problem, the float was being lifted, except the valve inside the float gauge was clogged or damaged and wasn't stopping the water entering the tank when it reached it's highest point, and then the water was just flowing through the overflow pipe and down the toilet.

So the simple fix was just to change the float gauge, they're all of about £10 at toolstation, and I fitted it with no problem

Hope this helps someone in the future