Best time to buy The One Plan Sim Only?

Hi all,

I'm planning on switching to Three in the next few days on their One Plan Sim Only offer. (£15 p/m for 12 Months)

However, I was wondering whether there was any chance of getting a better price or any discounts on it, if I were to sign up as an existing customer? (My Dad has 3 lines with them, and was thinking of getting the 4th as this providing it was a good offer).

Also, would now be a good time to sign up, or should I wait it out? Being a Sim Only plan, I am unsure whether it will decrease any more, which is the only thing I'm holding out for.

Thanks in advance!