Here is the EU regulation wording:
Right to care
1. Where reference is made to this Article, passengers shall be offered free of charge:
(a) meals and refreshments in a reasonable relation to the waiting time;
(b) hotel accommodation in cases - where a stay of one or more nights becomes necessary, or - where a stay additional to that intended by the passenger becomes necessary; ( c) transport between the airport and place of accommodation (hotel or other).
2. In addition, passengers shall be offered free of charge two telephone calls, telex or fax messages, or e-mails.
3. In applying this Article, the operating air carrier shall pay particular attention to the needs of persons with reduced mobility and any persons accompanying them, as well as to the needs of unaccompanied children.
I looked at the BA website on this and interestingly the wording is changed to NOT mention the 'free of charge' phrase. This means to me that you should not be obliged to pay and then reclaim any such costs, the airline must do this.
Easyjet claim that their policy is available at check-in desks.
Ryanair start out by claiming you get not a lot if the circumstances are beyond their control. However, they in fact MUST demonstrate EXTRAORDINARY circumstances, so bad weather wouldn't do! A bomb threat might!
BMI is slightly better but refer to delays within their control.
If you face such a situation you need to know your rights and the most important is that the airlines have to give you information and their compensation policy which must be in line with the regulation. However, it is clear that you need a copy of the regulation, not a re-written version from the airline which may be ambiguous.
The regulation applies provided the departure or destination is an EU member state.