Double check to make sure that it has at least a half day of direct sun. If less, you will need a special seed for shady places--there are several different brands. Get a good, rich, black garden compost (not potting compost) spread it about a cm thick in the bare areas, and scratch it in with a garden rake. Also apply some growmore fertilizer. Water deeply, in stages, to the equivalent of 5 cm of water. This will soak the growmore and the soluble part of the compost into the ground to condition the soil. Wait a day or two, so you won't be walking on mud, apply the new seed, and cover with 1/4 cm of compost. Water lightly, just enough to wet the compost, twice a day until the seed comes up. Then gradually water less often and more deeply, until the grass is living off of rainfall. I would give it one more feeding 6-8 weeks after the the grass sprouts, to encourage it to thicken up. Others would say not to feed until next spring, and I suspect that that depends on how long your growing season is--with our 12 month growing season and hot summers, we feed every 6 weeks like clockwork, though that probably won't work in the UK! : )