Labour's two tonne general election PR gaffe, mischievously dubbed the "Ed Stone", continues to haunt the party months after its disappearance. The limestone slab of key manifesto commitments was unveiled in the Conservative/Labour marginal of Hastings and Rye by Ed Miliband just days before the 7 May 2015 ballot.

The stone immediately attracted a barrage of ridicule from the commentariat and sections of the electorate. A hunt for the slab was launched after Labour's disastrous defeat to the Tories and The Guardian allegedly tracked it down to a garage in south London.

Interest in the "Ed Stone" was reawakened when the Electoral Commission published details of parties' spending during the election on 20 January. But the media could not find any mention of slab in the watchdog's records.

Meanwhile, Bloomberg broke the story that the slab cost almost £8,000 ($11,457) and revealed the piece was going to be originally unveiled in a school hall (it was too heavy and had to be parked outside).

Now the Electoral Commission has said Labour has provided it with the receipts for the plinth. But the watchdog has launched an investigation over what happened with the documents.

"Due to an administrative error, these invoices were not included with other items of the campaign spend," a Labour spokesperson said. "We have informed the Electoral Commission and will seek to rectify this error as soon as possible."