I would be inclined to agree with you, but unless they've got really really good elocution (?) lessons going on over there the girl spoke perfect English.
Turns out it is a bt number, and the girl I spoke to last night to finally sort it all out said that it was one of their call centres, which is a relief.
Think I've got off lightly with only £50 deposit, as even the bt rep on moneysavingexpert was reckoning on me having to actually pay for a new line.