You are effectively trying to use your pension as a Bank Account, making withdrawals as required. This is permissible under the new rules but not many Companies have agreed to provide this service so far from what I have seen in the papers. First thing is to contact them and ask.
Do you actually need the 25% Tax free up front, it is much more cost effective to draw down annual sums receiving 25% of each payment Tax Free.
Have a read of this link, someone has done the calculations.
http://www.thisismoney.co.uk/money/pensionfree/article-3020427/Why-SHOULDN-T-lump-sum-pension-amid-revolution.html.
HMRC have said they will be levying an emergency tax rate on all drawdowns at source, if they go ahead whichever way you draw it out, it looks like you will have to claim back any tax.