part repayment /part interest only paying off lump sum

I have a part repayment and part interest only mortgage which finishes in 3 years but will have a larger interest only part outstanding.

I have an endownment policy but it wont cover the whole interest only amount .

the figures are

current balance £46k

interest only £35k

I want to pay some off say £6k now does it come off the interest only amount or the repayment amount.
thanks and i hope this makes sense