It is probably moss, and I think you shouldn't seed for about 6 weeks (due to the weedkiller!). A bit late to give you this advice but ideally you should have treated it 6 weeks ago, as April is a great time to sow grass seed. It gets a bit too hot for good germination by the beginning of June.
Rake out the dead moss, but leave the seed sowing for a few weeks. If the patches are small in area you might try sowing after three weeks, watering well. If there is no sign of germination after 10 days then sow again. You'll just have wasted a few handfuls of seed. Once the seed has germinated you should hand weed for a couple of months as grass seedlings are sensitive to lawn weedkiller. idally you should not treat a newly sown lawn with weedkiller for six months.
I have just moved into a Bungalow in Leigh-on-Sea and what appeared to be a lovely green lawn on closer inspection when i moved in was covered in moss around the borders and other weeds elsewhere. I have treated it with WESTLAND 4-1 and whilst it has seen off the other weeds the moss prevails around the border. I take it that a strong weed killer in October will see it off and what is currently green with turn brown. I intend to reseed in April 2015. I would appreciate advice on this please.
A Jock who is unable to vote due to living in England as I have done since aged 16.
Ronnie, you need to improve the drainage first, moss grows in damp conditions so spike the area first by pushing a fork into the soil to a depth of 4-6 inches spaced every 6 inches and brush sharp sand into the holes. On it's own it may not be enough but it will improve the situation. You may need to repeat the process as only time will tell, so do it now and watch and wait. If you get moss over winter do the same again next spring then apply a moss killer. You may need to do it 3 or 4 times, but eventually you'll get on top of it.
I had the same problem here as the grass had just been cut and nothing else done to it, so it was largely moss and other weeds. Just doing a weed and feed last spring after a feed only, boosted the remaining grass and made a huge difference. Over the summer and autumn the grass became more established than the weeds as it was stronger. This year it's had the same treatment and it's now largely grass and very few weeds. The grass gradually spread into the previously mossy areas. Dave's right - improve drainage, apply some weed and feed and see what happens over the autumn and winter and then start again in spring with a similar regime. You may not even need to sow any seed. Scalping grass too short is also a common mistake - it weakens grass and lets weeds get more of a foothold, especially moss, so try to avoid that as well.