IS & mortgage interest payments

In my opinion the DWP would only be interested in considering " Intentional Deprivation of Capital" if your savings were around the £3000 mark. At this point benefit income remaining after the period for which it was paid, becomes capital and should, as far as the legislation is concerned, be used to reduce your benefits rather than reduce your mortgage.
While you savings and capital remain under the £3000 threshold how you spend your money is up to you. I feel it would be hard for them to argue "INTENTION TO DEPRIVE" when you are currently getting IS and Mortgage Interest payments and the effect of your action will decrease the amount of capital on which those Mortgage Interest payments are made, thus reducing the DWP contribution.
I've had a chat with my local office and the person I spoke to couldn't see anything wrong with what you are doing and so the message is carry on and stop worrying.