An update to this:
An engineer came around today and firstly started blaming my phone for not ringing properly, until I pointed out that I was having problems on *both* my lines!
After checking out various things (apparently nobody else in the area has reported faults) he said he'd get ntl to replace the cable outside my house, but the appointment wouldn't be until *next Wednesday*!
He then said that if I phoned up Customer Services they should be able to move this up sooner.
So when I phoned Customer Services the guy I spoke to said "I wish engineers would get on with their jobs and stop messing with ours because we can't change the date"!
The upshot of this is that I'm going to be stuck with dodgy phonelines for another *week* and them saying "oh, when it's fixed, phone us and we'll make sure you're not charged for that time" is no recompense (especially when I said "well if you know it's going to be next Wednesday, why not just refund me now?" to which the response was "well, it might not *be* next Wednesday if there's other problems...")
