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Are those leylandii?
@aggi if they are, get rid. If anything it will save you or a future owner the ballache of having them removed at 100' tall (ask me how I know).
As for the grass. I don't have the perfect answer but from bodging a lawn with seed, it's a matter of persistence and finding the right seed blend.
Get rid of the leylandii and you'll get more light. Then as soon as the season allows next year go into overseeing mode. Scalp it right back, aerate, sand, seed, top dress, water, feed, rinse and repeat and it will thicken out.
But, light is going to be your friend, ultimately.
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@aggi if they are, get rid
Iirc which is not guaranteed, this advice has been given several times previously.
I'll give it again. Get rid of those bastard things.
What do you use the lawn for? If you tripled the width of the borders, you'd be using up the muddiest patches and then you'd have enough room for some beautiful plants.
Are those leylandii? If you're not careful your whole garden will be nothing but leylandii soon!