Thanks for all the responses :)
SRS Microsystems is a Pentax Pro Centre(listed on Pentax's website as such
here) within a one hour radius from me, so they're reputable. They're also the cheapest, have a two year Pentax warranty, plus they stock a twin white kit.
They don't sell the DA 55-300, and other stores sell this for upwards of £250. New DAL costs £170 from Amazon, no signs of any used DA or DAL 55-300.
I'll be getting my dad to drive me to the store to buy(in person-- he's uncomfortable with such large purchases online with good reason) so I guess I'd be able to check when the time comes.
If the DA 50-200 is a DA, then you all say I should go for it?
(on a side note... how do I convince my parents that £500 isn't completely
too bad for a DSLR? ;) )