Buying used / secondhand gear is prob the best way to go, you will get a LOT more bass for you money, there is nothing worse than learning on something that is badly set up, sounds and feels nasty, just cause its shiny and new so there for it must be good.
With buying used (especially off a forum like this) the odds are that its been owned an played by a player who is enthusiastic about what there doing an the odds that he/she wasn't even the first owner of it in the first place.
Decent stuff is never relay owned by anybody, you buy it, play it, look after it an move it on to the next owner, the better the quality of the instrument in the first place the longer the life of it (if i was wrong why do vintage guitars & basses sell for silly amounts of money when in effect there old an worn out ?? i think not) but we are not talking vintage or even pro gear but for the new budget you have of £400 you have just opened up a new door of options, and as a bonus a used bass thats a couple of years old, seen i few gigs, is played in an sounding good, say you decide that 6months down the line you don't want to play anymore then wen you come to sell it on expect to get back pretty much what you payed for it initially.
In other words you wont loose anything on it might even make a few quid on it when you sell it.