Your best bet (in my view) is to clear out everything you dont want, and start afresh. The lawn will probably be completly shot, and no amount of weeding and feeding will make it into a decent lawn. And any clearing of current weeds will simply be replaced by new ones as you expose the soil as those seeds currently lying dormant will spring into life. If you take off the current turf, you will also be taking away the soil which has been scattered with years of weed seeds. Some seeds can survive up to 50 years in dormancy!!