Save Income Tax from a Buy

[chance]

I have a Buy to Let, flexible mortgage (i.e. I can make overpayments and withdraw any capital I've put in whenever I want). So far I've only used this to put in the occasional lump sum.

The actual mortgage payments each month are well under what I receive in rent, and so far I've been keeping/spending the excess like normal wages.

It's only just dawned on me how much income tax I'm paying on this excess (it takes me into the higher tax bracket), so I'm thinking of remortgaging so that the payments match rent I receive, and ok I lose out on my extra 'spending money' each month but in the long run it's more tax efficient as I can write the whole rent off as a necessary incurred cost.

But, I'm wondering if I actually need to remortgage - could I just put the excess money into the flexible mortgage as overpayments each month? I'm not sure whether the Inland Revenue will buy it when I come to declare it as a necessary incurred cost as it has been my choice to make the overpayments, therefore is it really a cost?

Also - I've just realized that I could theoretically then draw it all back out the following tax year, having avoided paying income tax on it. What's the rules on this? Have I missed something? Am I being stupid????!

Hope that all makes sense - any advice appreciated!
Thanks
chance