Hello Mikey and welcome to the family. Glad to have you.
Be forewarned I sometimes a come off sounding bit abrupt. I mean no disrespect. Just sometimes direct is the quickest way to help out. There is oil inside that smoker. Not good oil. Manufacturing oil. Build a fire in the firebox AND the cooking chamber You want the fire to burn off all that oil. I did two burns in mine. One to burn off the nasty oil and a second to season the smoker. For the second burn you can just build the fire in the firebox.
This is a pre-written response for inexperienced new folks but if you don’t fall into that category, you might still find an idea or 2. You need to do a few modifications to that smoker to help with temp control. Without meat, get a fire going in there to create smoke. Small fire, BIG smoke! You can even spray a little water on the coals to create BIG white smoke. What you want to do is see where the smoke leaks are. Mark the leaks, open the lid and allow the fire to burn down or go out. When the smoker cools seal every leak you can using stove rope, high temp silicone, bbq gasket and such. Next, if you have a thin flimsy fire grate use it as a template and build or have built a grate out of 1/2" concrete reinforcing steel ( rebar ). That thin grate will sag with heat and will rest on the ash cutting off air flow to your coals. No air flow no heat. Other option is build a charcoal basket. You can find baskets in the build section. Leave that exhaust fully open and use the intake vent to control the heat. Last tip is go buy a cheap garden trowel. Knock the wooden handle off and weld a 2-2 1/2' piece of that rebar to the shovel. Now you can gently scoop out the ash without them blowing all over your meat and you won't burn your hands.
The next thing is use the BigK to season the smoker and NEVER buy it again. It is a waste of money. It is cheap for a reason. We will help you find good charcoal. You will pay more but you will use less and turn out a better end product.
Please don't think I am "barking orders". I am really not as grumpy as I come across. I just want to help you get started in the right way. I don't want you to fail first time out and then loose heart and give up. I don't want to lose you at the start. We can do this together and the other U.K. members will be there to support you. Great bunch of folks who know their stuff. Keep Smokin!