Pay rise was £0.11 (wow, still... money is money) Before the increase it was 6.30, now it's 6.42, only a small increase, but if the employer wished, they could they have just increase me rate of pay to 6.31 with the NMW increase, forgetting the bonus, simply based on the fact that £6.31 is higher than my old rate?

I recently received a pay rise in work bringing my hourly rate of pay above the old national minimum wage. I am aware that the minimum wage has recently increased. If my hourly rate with pay rise before the NMW increase was lower than the new NMW (£6.31), is my employer allowed to simply pay me £6.31, or must the new NMW also see the pay rise? And are employers allowed to revoke an increase in pay?