Hi Allys,
As has been posted before, the agreement in principle means that that have provisonally agreed to provide you a mortgage, based on the assumption that you will meet all of their lending conditions, and on they will perform suitable credit checks and references upon your submission of a mortgage application which will contain more information than they have at the AIP stage.
This application typically costs you an application fee, and then a valuation fee.
Unless the mortgage company advises you incorrectly these fees are not refundable if you are not sucessful in your application.
Are the AIP's worth the paper they are written on? No probably not.
They are a useful tool for getting to view new properties to buy with an indication of the possible lending and should stop other advisors attached to agents pushing to arrrange a mortgage for you.
If you want to find out more a search on any lender's website should explain what an AIP covers.
A quick search for the abbey AIP thread will also give you a useful link.
HTH
Sparky.