Unhealthy grass. Not the smoking kind!

[Stars66]

I was told to get some Evergreen for our lawn, which is a mixture of almost dead grass, a bit of moss and the odd weed. I've bought extreme green lawn food, but DH is laughing at me as he reckons the grass is what it is and I've just chucked away £18 although he hasn't noticed I got it on his cc Anyway I bought grass seed too.
What should I do first? Try and improve the grass, or grow new stuff on the bare bits?
And does this evergreen stuff work?!

Evergreen 4 in 1, verdone or Wilkos equivalent. You need to tackle the weeds, moss and the grass. Don't just feed the lawn, it wont eradicate the weeds etc. After treating the lawn, allow some time for it to work.
If no rain comes it will need watering ( see instructions)
You will notice a lot of black spots; this where the moss has died off.
Don't be tempted to feed the lawn after the treatment is complete. Regular mowing and raking are all that's needed.
You can treat the lawn again in the Autumn with a formulation to promote root growth.
Any bare patches can be sorted by sprinkling on some lawn seed or removing dead turves with a spade and replacing with a patch of fresh turf.

Ah thanks, it looks like I've bought the wrong stuff then!

Don't throw it away return it to the shop if you have a receipt or keep it for when the lawn has been eradicated of moss and weeds and needs a boost, but only in the spring or summer.

Yeah will do. To be fair, I made my garden sound awful, there's a little bit of moss around the edge where our bushes blocked the light (were overgrown and are now cut back), and the odd dandelion which I dig up but obv never dig deep enough! The grass is just looking threadbare (can grass be threadbare?!) and thin, and I've just laid a bit of turf at the other end which makes the threadbare bit look worse!

Spray bicarbonate of soda on the moss, once it turns black rake it out. For the bare patches I would wait until it's not as warm then overseed the areas, run a rake over the ground first then mix the seed with some sand or topsoil and water it. Treat the weeds with a normal weed killer for lawns, there are lots about to choose from.