As you rightly say, maintenance is paid net of tax, so at the moment you receive your lump sum (when nothing has been done to it), it isn't taxable. But when calculating the lump sum, solicitors will base it on your net-of-tax income requirements, using something called The Duxbury Tables to work out what you'll need.
These tables assume you will invest some of your money and spend some - any income you receive from these investments will bear tax.
Solicitors will also use a process called 'capitalisation' when calculating a lump sum. When a maintenance order is being capitalised, there is often an uplift on the maintenance figure, so it is important to ensure the base figure is correct.
You also need to check the court order with your solicitor to see whether the capitalised sum can cover whole of life instead of up to 2017. This may be possible if the original court order does not expressly prevent you from applying for an extension of the term past 2017.Take legal advice as all of the above is complicated.