Until PlusNet release full details of their FTTC packages in April (there are likely to be specific packages for FTTC, some with higher allowances and all with higher costs), the £12 figure is an interim cost on existing packages until it is all announced. After that you will have to change to a FTTC package to get FTTC.
Also you will need to be on one of Plusnets current products (Value, Extra or Pro) to get FTTC at the moment, tral or otherwise.
The last date to apply for the trial was 25th Jan so if you have not already applied, you can still do so but you will have to pay the additional £12/month cost and also move to Value, Extra or Pro. Those on the trial or already applied will get FTTC on their existing package with no additional cost.
More details on the following 2 plusnet Blog posts
Fibre to the cabinet trial ends soon
Plusnet fibre trials update
I've been on the FTTC trial since June and it's been very reliable and VERY FAST!... and not a DLM in sight