Long(ish) line query

Looking to order BB on a business line. Main requirement is stability, rather than speed.
BT Checker reports only 512k available and possibly 1meg if MaxADSL is ordered.
The line has never had broadband on it before, so I guess the checker could well be wrong. Other nearby properties with broadband seem to report 1-1.5meg on the BT checker..

Is it still possible to order fixed rate circuits with Zen? I'd prefer to get a 512k fixed and then regrade to Max if the actual line stats look reasonable.

Zen have a Zen Office 500 available here http://www.zen.co.uk/Broadband/ML_Business.aspx?page=519
but it is rather pricey because it is unlimited.
They also have a 250kbs service available here http://www.zen.co.uk/Broadband/athome.aspx?page=453 which might be a better bet for testing your line.