Big money should never be allowed to decide the results of elections or referendums but, equally, campaigns do not come free.

Posters, websites, television adverts, travel and all the other ancillary expenses of running a campaign have to be met somehow and, as long as state funding of electioneering remains politically unacceptable, the bills will have to be paid for by private individuals and organisations.

For that reason, there is nothing wrong in principle with the Harry Potter author JK Rowling donating £1 million to the Better Together campaign, just as there is nothing wrong with the Lottery winners Chris and Colin Weir donating £3.5m to the Yes campaign. Ms Rowling is also putting her money where her mouth is and has eloquently explained why she believes Scotland is better in the Union.

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There will be many who disagree with her arguments and, indeed, there was a virulent and unacceptable reaction to her comments from some nationalists online yesterday. But what both sides can agree on is that big donations from the likes of Ms Rowling and the Weirs have to be part of a regime that aims to prevent any side gaining a significant financial advantage.

To this end, Scotland is now in an official campaigning period that began on May 30 and restricts what both sides can spend to £1.5m each. This will help make the process as fair and transparent as possible, although it is complicated by the fact that individual parties can also spend on the campaign, up to set limits according to their representation at Holyrood. Unfortunately, none of this means we know precisely how much has been spent in promoting both the No and Yes sides. Taxpayers' money, for instance, has apparently been spent on both sides with Alistair Darling, the Better Together leader, complaining that the Scottish Government used taxpayers' money to promote its White Paper, and the SNP complaining that money has been spent on material prepared by civil servants in London favourable to the No cause.

On top of this, many relatively small donations have been made by private individuals on both sides although there are rules that apply here too. Unregistered individuals and groups are limited to £10,000 (and risk a fine if they disobey) and registered groups to £150,000.

The cumulative effect of all of this is unclear, and it may be that one side will end up spending much more than the other, particularly as there was no total upper limit on spending before May 30. Regardless of the final bill, both sides must commit themselves to sticking to the limits and also being totally transparent about what has been spent.

In the words of John McCormick, Electoral Commissioner for Scotland, voters must be confident that the campaigners are playing by the rules but voters also know that the referendum has to be paid for and that must be done fairly. If continuing rancour and enmity are to be avoided after September 18, no side should be open to the accusation that the result, whatever it may be, was "bought" by wealthy individuals.