ok,
WHS = What he said (liitle bit sexist but generally on here it's mainly he with the odd feisty she and a lot of fembots created by others)
1st tip - always read your contract
2nd tip - ALWAYS READ YOUR CONTRACT
From experience contracting is a tough game as in most cases we (the small 1 man band or slightly higher) against them (the multi national corp's) who have no intention or care how much you pay if the FD says dont pay just yet they dont. This results in the very fast growing of a tough skin and the only thing you have in defence against this corporate bullying is the flimsy bit of paper with the terms and conditions written on it and after a while the comfort in knowing you can live a year or more from your current bank balance if need be. That should explain the "i've been through it myself, learnt the hard way and so why should i help you wipe your arse if you dont do some basic research first" style responses you've been getting, they're all cuddly bears mainly (some slightly greener and alien liek than others)
From what you've said and reading the response you may well be a "contractor" in the eyes of your agency, if you go through an umbrella then your effectively an employee of the umbrella rather than fending for yourself.
Read the Free Stuff section here ----------------------->
As I see it you have 3 choices:
1) do nothing and continue as is, maybe moan on here about doom and gloom along with most of the others, if so welcome

you'll fit right in, maybe grow a thicker skin first though
2) negotiate, what have you got to lose?, for an increase in rate to offset the holiday deduction
3) form your own limited (nixon williams and SJD accountancy would be highly recommended from most on here among others as a starting point) and get rid of the umbrella, you'll then be in charge of your companies finances and can therefore pay yourself whatever you like along with every so often having a great in depth chat with yourself about what payrise/dividend/laptop/company mobile you believe your worth.. always an entertaining chat, especially if it involves brownie points with the Company Secretary (Mrs Pearson i'd presume) while having a "Company Meeting" in a posh restaurant (all expensable.. read the guides -------> )
either way HTH (hope this helps

)
Chef