I spoke to Justin at Citibank 132484 on 15 Jan 15 at 10:40am who informed -

No 'Interest Free Period' on 'Purchases' during tenure of 2 years 'Balance Transfer' @ 0% interest.

If Credit Cardholder fails to pay Total Amount Owing or indeed fails to pay Minimum Monthly Payment (usually between 2˝ and 5%) of Total Amount Owing the Credit Cardholder will incur 'Late Payment Fees, but not forfeit his 2 years interest free period on his Balance Transfer.

*Above Savings based on balance transfer for a period of 24 months at an average interest rate of 20% p.a. Assumes minimum monthly payment made as due, interest charged monthly, and no other transactions during period.