Well as some of you know I recently found a DT125a with a CT1 175 engine in great original condition. 
Today I rode it for a small distance (14 miles) into work for the first time and when it warmed up it started to hold back then pop and finished in losing power. 
I thought for a minute that I wasn’t going to make it however I still need to get home, lol. 
Now it’s cooled down I tried to kick it over and it’s just popping and ‘fluffing’. 
To me it sounds like fuel unless someone can tell me different. 

Any ideas? 

Sometimes’ popping means the stator is going (shorting) out. The bike could be running too lean? Many things.

a

I’m running it at 40/1 as recommended therefore 125ml of oil to 5ltrs of fuel. 

it was running fine and then started having issues. I’ve just tried kicking it over and it is just popping and spluttering but won’t start now. 

Oil pump not working?

Vapor lock? Loosen the gas cap. Vent might be blocked. 

It’s mixed in the tank. 

Ok. 100% guarantee its shorn off the flywheel key from having been off & not tightened sufficiently. Make that 50% guarantee & give myself a bit of leeway incase i'm wrong haha.

Need to pop the plug to get an idea of what's been going on and see if you still have a spark.
Mag side crank seal could have pushed out, happened on my CT1-B.
Could be a lot of things really, need to investigate.

"It’s mixed in the tank." 

Originally there was an oil pump, situated under the small cover at the front right hand side of the engine.

Oil tank is/was the tinware on the left side of the bike, accessed from under the seat. Has all of this been removed? 

Edit - just had a look at your photos. Oil tank is still present, but I can't see any hoses from the tank to the pump or the pump to the engine, hence your running it on pre-mix.

Hi all, I managed to get it started again tonight but won’t idle. I’ve left the bike at work and will take some tools in tomorrow to investigate further. 

once started I pulled the choke and it tried to die by suffocation, the same as what’s happening therefore I’ll remove and clean out the carb and see if that helps.