"BT say they might get 512k after an engineer's
visit"
that's a fairly standard line, "very unlikely" is the message on the
longest lines. There will only be an engineer visit if you can't make
it work, you request it via the ISP.
Home 250 from Zen with a BT Voyager 105 or a Vigor router (Zen will
supply that and refund if the line doesn't work out) would be the
prescription on a "very unlikely" line. Vigor do "long reach firmware"
but I have no evidence its any better.
The main thing is to plug into the BT test socket behind the faceplate
http://www.clarity.it/telecoms/adsl_faceplate.htm#part3 to eliminate
internal wiring as a potential cause of interference or other problems.
That's what a visiting engineer will do, if it doesn't work there he'll
try to improve it by checking the line for faults or trying a spare
pair (if available).
Phil