Most ISP's determine this by using the BT Wholesale phone number data base. You can try it yourself to see if it ties up with what Tiscali are telling you:
http://eco.btwholesale.com/broadband1/
Some things to bear in mind:
It is usually fairly accurate, but has been known to be wrong. If it is wrong getting it changed can be hard, but it is possible.
Because the property is nearer the exchange than yours does not neccessarily mean it will be faster. Maybe the cable is routed half way round the town before it gets to your daughters (it does not always take the shortest route). Maybe the cable is poorer quality than the cable that links you etc.
It is only an estimate. If ADSL max is being installed it will connect at the fastest stable speed possible for the line.
Maybe she is not connecting to the exchange you think she is. Believe it or not you do not always get connected to youur nearest one. You can check this for sure at Sam Knows:
http://www.samknows.com/broadband/checker2.php
After you put in your details. click on locality.
Once you get a broadband service up and running on the line you can then get some data from the router/modem that will tell you what speed the line is really capable of. Armed with this infor a good ISP may be able to get you onto a faster service.
On a personal note I would not use Tiscali on a line like this. Sounds like you may need to use support quite a lot, and if they turn out to be of little help you do not really want to be tied into a long contract.