BSSD "Very very strange as Plusnet and BT are part of the same group of companies."
Plus.Net was independent until about 6 years ago when it was bought by BT. It runs completely separately from BT and that shows in both products (mostly cheaper) and call centre (in Sheffield for all enquiries, has an 03 number for use from mobiles, 08xx from landlines).
PN has been left alone, as far as I can tell, although it does adopt some of the same approaches...
If you take Plus.Net's landline rental, then like BT, if you pay in advance for a year (to get the charge down to under 11 quid) you get no refund if you decide to switch.
BT started that, when OFCOM made telecom firms drop the cancellation charges (a large chunk of the monthly fee is paid to Openreach [BT] for the line (maintenance, etc)).
If you switch from say Sky to TalkTalk then Sky is no longer paying 8 quid a month to Openreach so OFCOM said they could charge only their costs (around 3 to 5 quid a month, not 12 to 15). So nowadays, cancelling line rental from some telecom firms (and having to pay for remaining months of contract) is a fraction of what it was, but BT (and now Plus.Net and possibly others) will just not refund any part the 'year in advance' fee...
A bit nasty, but most firms are requiring 12 or 18 month contracts nowadays, and you are hit hard if you go monthly with many.