Line tests fail for various reasons and not always because of a line fault. Sometimes the line test fails within diagnostics which can mean that the test equipment was busy or faulty!
O2 CAN book an engineer without line test failure but it will become a chargeable visit if the engineer cannot find a fault or gain access. They would at this stage try and pass on the cost of this to you, in some cases with a nice little admin charge on top! They should still offer you the option of a site visit. You just have to be persistant!
Whenever i request a line test through BTW it always feels like shaking one of those Lucky Eight balls if you know what I mean 
I'll stick my neck out here and suggest that you're talking at cross-purposes.
A 'Line test' is a simple low-frequency one shot test, that basically tests the pair of wires from the Exchange to the NTE5 at approx 1.6Khz. This takes approx 3-4 minutes to achieve, and if indeed the line is busy, or the test equipment has locked up, can be restarted again very, very easily.
I would like to think that Rocket Rons line has already undergone this simple test, as it is protocol to do this whenevr an EU complains about their service, be that PSTN or DSL.
I suggest that what RR meant to say was that his line was going to be monitored over a 24 hr period (or longer). This is in a way, a 'test', but not the one I think you are pertaining to Ottersnose.
If I'm wrong pal, then it's a damned crying shame that the ISP has not managed to attain ONE single line test !!!
