If you were shooting for a 1.048 and had a 1.050 pre-boil I wouldn't worry too much about the speed of your sparge. Personally, I batch sparge rather than fly sparge so I never have to worry about the speed of my sparge. And the last bit of wort is always going to be thinner than the first runnings, so thats not at all strange.
Anyway, at some point in time you will need to figure out your brewhouse efficiency, because if 1.048 is what you were looking for as the OG, you probably ended up in the mid to upper 1.050's after your boil. Not nearly as far off as my first attempt at an ESB, where I shot for a 1.058 and ended up with a 1.085.
By the way, you can calculate your efficiency and figure out what you OG would be using some of the calculators on the same website as the link I posted.