On 5 May, the whole of the UK goes to the polls to vote in a referendum for the first time since 1975. Voters will be asked to decide whether they want to replace the existing "first past the post system" to elect MPs to the House of Commons with the "alternative vote" system.

It is a referendum that will see some unlikely alliances forming on either side of the campaign, with some the UK's largest trades unions lining up alongside senior Conservative party politicians to push for a "no" vote, while those advocating a "yes" come not only from established parties like the Liberal Democrats and some sections of the Labour Party, but also smaller parties like the Green Party.

But with such a complex mix of interested parties, how much do we know about who is bankrolling the campaigns and what their agendas are? Reporter James Silver investigates the campaign groups and private individuals pumping millions of pounds into the contest and asks whether the rules around disclosure of donations are as robust as those for general elections.