FMQs: Scottish Conservatives leader Douglas Ross left red-faced after turning the Holyrood air blue

What is it about microphones and politicians?

During the 2010 general election, Gordon Brown described a voter as a “bigoted woman” not realising that the radio mic on his lapel, pinned there courtesy of Sky News, was still in record mode.

He immediately had to revisit the home of Mrs Gillian Duffy, who found herself high in the celeb stakes for a full day of election campaigning. Brown offered his apologies in tones that tickled maximum cringe.

Journalists just love it when technology trips a tribune into revealing their true feelings.

Closer to home, the former Labour MSP Frank McAveety gave a short commentary on the pulchritude of a young woman at the back of a parliamentary committee room, not realising his microphone was live and recorded his “dark and dusky” comment.

He was mortified to find out the subject of his affection was a 15-year-old schoolgirl. He resigned from the committee.

Now it seems that bad language follows the Scottish Conservative leader Douglas Ross,

Whilst officiating at a recent football match in the east end of Glasgow, the ultras group The Green Brigade unveiled a banner containing language that must have made Ross wince.

Well today, it was his turn. He declared that he was “on a roll” as he laid into the government over the introduction of the deposit return scheme.

In fairness, he was on a roll with some rather good lines, but his momentum was interrupted by yet another shouty protester in the Holyrood gallery.

As he slumped to his seat following the intervention of the presiding officer, he could be heard exclaiming “for f***’s sake”.

The exchanges at FMQs were suspended in order that the offending protester could be removed. When they re-convened, Ross issued an apology for his “industrial language” and seemed particularly perturbed that his mother might have heard his expletive.

When the session ended, he will no doubt have kicked himself that he made such a schoolboy error in not realising his microphone was not mute.

The First Minister could not help but observe that perhaps he was indeed on a roll, but one that was downhill.

Ross had interrogated Nicola Sturgeon with a simple point. Yesterday, the minster in charge of the introduction of the scheme, the Scottish Green MSP Lorna Slater, could not answer a simple question: how many businesses had signed up to the scheme?

In all my years of observing politics, I have never encountered a minister as thoroughly unconvincing in a parliamentary chamber as Slater.

She doesn’t need the astuteness of clever opposition to reduce her to a nervous wreck style of delivery. She seems to manage that all by herself.

It’s as if the voluminous civil service notes she clutches as a sort of debating life jacket don’t include the lines, ‘don’t choke on your words and don’t shake as you deliver them’.

Ross, and later the Scottish Labour leader Anas Sarwar, derided the handling of the deposit return scheme.

The scheme adds 20p to products sold in a single-use container. The 20p is then paid back when the container is returned, thus hopefully cutting back on litter and emissions.

Sturgeon said the issue here was not the number of businesses that had signed up, but the volume of the market included in the scheme.

She repeated the line of her junior minister from the previous day, namely that producers who account for 90% of such containers had signed up.

She conceded that although 664 business had got on board, the number of businesses affected could be as much as 2,000.

Ross read out some scathing comments from businesses accusing the First Minister of being in a hole and continuing to dig.

Of course, the exchanges are part of the First Minister’s long goodbye and both Ross and Sarwar are keen to point out the alleged shortcomings of the three hopefuls aiming to replace her.

The SNP leadership contest is now in full swing and, until it is resolved, will consume Scottish politics and relegate the bread and butter issues that so dominate voters’ concerns.

That contest will take off in earnest when the three candidates face one another in a live STV debate next Tuesday at 9pm.

I look forward to it. Stay tuned for my verdict on what the contents of our programme mean for the SNP and the wider debate on the country’s constitutional future.