craig, you still looking?
and you wanting Payg sounding by ur budget?
much you top ur phone up each month? (much £ you topup a month?)
but on the phone side of things,
D500 - is getting abit old now (still good dont get me wrong, but oldish) 1.3 mega pixel camera, good music etc. would go with something more upmarket so you can sell it for something decent when you get a new phone
D600 - iv been told by so many people they dont think its as good as the d500, dunno if this is because, the whole slide thing has been worn out, since the e800 and d500 have been out.
K750i - great phone, i would say that as i have one, but geenraly best phone iv had. no problems at all, sounds great, pictures are great (auto focus 2mega pixel) and surprisingly wasnt too hard for me to learn how to use, as i expeced a massive difficulty after having my Nokia 6230.
W800i - same as k750i except comes with a 512mb memory card instead of the standard 64mb. depending on which you prefer the whole walkman orange and white look or the k750i's black and silver stylish look can make a big impact in which you prefer.
these phones are so like each other, the cases for them fit each of both phones, (so you can take a w800i cover of and put it on a k750i and theyll fit the same). Also theres a few lil different features which are basicly because its a walkman phone, such as you can play music on it without actualy turning on your normal phone (you cant accept calls or anytihng, basicly makes it just a normal mp3 player) good for on flights?
But bare in mind, new phones come out all time, i would suggest A contract phone if your spending quite abit on ur monthly bill on payg
PAYG =D600 = £300? + £30 x 12 = £660 (thats if you put £30 on ur phone topups on payg a month)
Contract = £40 x 12 = £480 (thats if you go on a £40 contract a month, and get the phone free obviously)
you wouldnt even really need to go on that high of a contract, depending how much you spend on your phone theres a £180 saving already

(over a 12 month period on both PAYG + Contract)
Hope I Helped, Tofas