SA Payments on account

What happens where an underpayment effectively means your payments on acount should have been higher ie: for underpayment interest purposes?

Eg :
Jan 01 - 1st on acct £1000
July 01 - 2nd on acct £1000

Say, before July 01 you know your final tax bill will be £3000, would you HAVE to pay the £2k with your Jul 01 payment OR pay bal of £1k in Jan02, thereby obtaining cashflow advantage. Any interest chargeable on shortfall ?
RT