Unlimited short term ISP

My girlfriend is on a BT only exchange but there is the possibility of FTTH as part of a community run project in the spring, in the meantime is there an 'Unlimited' BTW provider that will do a 3 month rolling contract?

Off top of my head no, most unlimited have a 12 month contract

PlusNet Pro may suit

http://www.plus.net/addons/pro/

Is the exchange still 20CN? (reads like it could be )

Which exchange is it?

Yes its 20CN

The exchange is Hornby and there's no LLU presence .

Don't think you will get "unlimited" on a one or three month contract from an ISP using BTw, how much usage is actually needed per month, there's an ISP who provides 100 GB + Free + unmetered usage 11pm - 9am and that is Eclipse but it ain't cheap.

A friend in the village (20CN exchange) uses another package from Eclipse and I've not heard any complaints from him, the GB usage isn't heavy.

http://www.eclipse.net.uk/home-broadband/products-an...

Lots of Netflix usage.

Lets forget about the short term aspect of my request...

BT Total + Line Rental comes to £39.40 a month, is there any unlimited providers that can trump that?

If it's of any interest the Post Office provide an unlimited BB package with a FUP, here's a link to their website.
I'll leave you to read and compare the details