Hey loz. Saw this thread last night and it gave me more of an idea what your situation is. I took my time to write this out so I hope it helps.
The setup you have will be on the lower end of okay. A lot of light leaks. However it should be fine. If you browse amazon enough you will find a grow tent that's around 100x100x180 for roughly £35.
The medium you are using is soil with perlite, soil holds water a lot more than coco, when I start my babies off I pre water my medium(bio bizz light mix) with plain ph'd water (3 litres) in 3 gallon buckets. I pop my seed in the rooting cube (Root riot - £2 a pack of 24) and pop it in the middle of my bucket. LEAVE THE F*** ALONE for atleast a week, when pot is dry go again with a watering and leave until dry. Maintaining a good wet/dry cycle is imperative. The light mix soil has little food in there which means no added nutrients for around 2/3 weeks (around 4-5 in all mix). You need a fan, the idea is to create an environment similar to the plants natural grow, the air cycled by the fan will help your plant stems get stronger and sturdier. Once your plant hits 3/4 weeks old (this is for an autoflower not a photo period) it will start to show sex, possibly starting early flowering process, you'll see a couple white hairs sticking out, this means it's time to start adding nutrients. I always recommend for your first feed do it at half strength and if no problems go to full. Personally, I skip the early flowering recommended amount and start straight on the mid flower guideline. By now you should of noticed your plant drinking twice as fast as before, this is a good thing. Your leafs should be nice and green still, if any yellowing occurs 9/10 in soil, you will need cal/mag, not more nitrogen. The rest is pretty straight forward from here. Things I forgot to add. Some people say you don't need to PH water in your soil, my water comes from tap at 9.0 so I definitely pH mine and I would anyway just to be sure, takes 1 minute. Soil is more prone to pests, keeping a good wet dry cycle will 90% keep them away. Also, I hope you don't mind smell, I can see you have one plant but oh boy will your house smell, without a carbon filter i would recommend burning a Citronella candle in your room (also helps keeps bugs away) if you don't want the smell. Also, adjust your light. I keep mine on 600w boosted mode and have it 26" away from my plant. For 250 yours seems too far away.