won't be beat on price

[#1]

Plusnet will let a new customer get the phone and broadband service for £3.24 a month,  BUT  a return "loyal" customer is charged £12.99 a  month
  Are there any customers who stay for more than one year,,,well you must be daft........

[#2]

It is "daft" Valerie but sadly a fact of life almost everywhere. It's done to "hook" you and hope you'll not go elsewhere when it's time for renewal, I think the term for it is "inertia".
I received a home insurance renewal this morning which I'll not be paying as an existing customer. I have taken out an exactly similar policy with the same company, as a new customer, for £20 less!

[#3]

The 12-month half-price broadband offer for a new phone + broadband customer in a market 1 area is £6.49 rather than the eye-catching £3.24 which applies in markets 2 & 3. Clearly this is to attract new customers, but then, as you say, they need to be kept.
Rather than feeling aggrieved that only new customers get the good deals why not give Plusnet's Customer Options Team a call on 0800 432 0200? You would need to call weekdays 09:00 to 17:30 but they might be able to offer a deal to your liking to keep your custom. You can do this at any time, no need to wait for a joining anniversary.

[#4]

And as spraxyt says you do need to make that 'phone call to achieve any reductions. I only received the £20 insurance premium reduction by doing that. No threats to go elsewhere, maybe a "hint" though,just be pleasant, polite, state your case and it's surprising what you might receive in return.
There will, quite probably, be a minimum offer depending on the exchange. Mine is Market 1 but there are two LLU alternatives to BT Wholesale available there which did help considerably in my "negotiations"! However,  of those LLU providers one I will not touch, from experience, even with a 10ft long disinfected bargepole!