Get these guys out, save's you all the effort with the scarifier (it's HARD work to do it properly)
http://www.greenthumb.co.uk/ Use a cylinder cut mower instead of a rotary, and as monstermob's said, only take 1/3 of the length off each cut. It's a pain but worth it, during the summer months 2 cuts a week is normal if you want a lush tidy lawn.