Penny Mordaunt was at her happiest when she was Defence Secretary. A Naval reservist, the daughter of a Para, she was born to the job. So it was pleasing that on Tuesday she offered the House of Commons a textbook demonstration of that important rhetorical manoeuvre, the “Motte and Bailey defence”.

For those whose school years didn’t involve repeatedly studying the feudal system, the motte and bailey was a form of early castle, where a highly defensible keep, the motte, was surrounded by an expansive but harder to defend area, the bailey.

Mordaunt had her fortifications laid out as she arrived at the Commons, standing in for Michael Gove, to discuss the enforcement of the Ministerial Code. This is supposed to ensure that the government behaves with integrity, but Boris Johnson takes the Pirates of the Caribbean approach, that the code is more guidelines than actual rules.

Mordaunt was all politeness as she opened, a perfect deputy for Gove, a man so studiously courteous that he once had to employ Dominic Cummings to insult people on his behalf.

Angela Rayner, holder of the thousand job titles, conqueror of Keir’s office, she who upholds the leader (for now), was speaking for Labour. “Time and again ministers act like the rules are for other people, none more so than the prime minister himself,” she said.

She began to ask about Johnson’s Mustique holiday, but Speaker Lindsay Hoyle intervened, pointing out this was under investigation. When she instead asked about the curious employment arrangements of Eddie Lister, Johnson’s former chief of staff, Hoyle explained that these too were off limits, as Lister sits in the Lords. What, Rayner wanted to know, about the latest suggestions that people who had a Cabinet minister’s phone number found it a lot easier to get an NHS supply contract last year than people who didn’t?

Mordaunt rose to reply. The trick with the motte-and-bailey defence is to initially offer a vast and expansive claim – this is your bailey – and then when someone points out that much of it is rubbish, to retreat to narrower, much more defensible claims – your motte – and hope that no one notices the difference.

So Mordaunt wasn’t going to claim that Johnson was innocent. She’s not an idiot. She’s as aware as the rest of us that the prime minister’s approach of refusing to cooperate with inquiries does not imply much confidence in acquittal.

Mordaunt wasn’t going to claim that Johnson was innocent. She’s not an idiot

Her approach was to deliberately misunderstand Rayner. “The charge she makes is that the people she names are somehow on the take,” Mordaunt said. “That they have, unbelievably, entered into politics, made sacrifices, overcome the obstacles that she will be aware of to get into this place, not to serve in public life, but to do a mate, or more accurately a Tory mate, or someone that they vaguely know or met in a lift once or perhaps don’t know at all, a favour.”

This was not the charge that Rayner had made, but Mordaunt wasn’t done. “If you were to take every single MP she has made an allegation about this afternoon, if you were to look at all the political donations they have received since the pandemic started, since January 2020, and if you were to add them all up and then double them—no, quadruple them—you would just about match what the Right Honourable Lady herself has received in the same time period,” she said.

This was greeted with delight by the Tory benches, though readers might note that the language was extremely specific, covering a period when Rayner was fighting in a leadership contest, and none of the Conservatives in question were. A more interesting point of comparison is that the sum involved, around £225,000, represents roughly half of what Johnson was paid for speaking engagements in the months before he became prime minister.

Mordaunt finished, her forces triumphantly occupying the bailey, very pleased with themselves. But then Stewart Hosie spoke for the SNP. What, he wanted to know, about the 57 messages that David Cameron had sent to “Gove, everyone” on behalf of Greensill?

Suddenly, Mordaunt was back in the motte. “These issues are being looked at,” she said. “And it would not be appropriate for me to comment on those until they have reported.” Was it safe to come out yet?

Apparently not. There were more marauding Scots. Alistair Carmichael for the Lib Dems wanted investigations to be independent of government. It was all quite tricky, Mordaunt replied, shouting through an arrow slit in her keep.

Scott Benton, Tory MP for Blackpool, offered some respite. Weren’t ministers right to have done everything in their power to secure protective equipment for hospitals? Out charged Mordaunt. Yes, they were. Some people had criticised them, but if it was a crime to love hospitals, then by golly she would plead guilty. Another Tory, Jerome Mayhew wanted to know if Labour was being “deeply irresponsible” even to ask about this stuff, and Mordaunt agreed that they were.

But here was Angela Eagle, asking how Johnson could judge ministers when he didn’t respect the code, and straight back to the motte went Mordaunt. “The prime minister does respect the ministerial code and he is the arbiter of it.”

It was impressive to watch, and it kept the troops happy. They enjoyed the bits where Mordaunt was in the bailey, and they stayed quiet whenever they had to run back to safety. They even offered Labour some advice, like Saxons leaning over the battlements and yelling to the Vikings that they’d be a lot more popular if they cut out the marauding.

The battle ended in a stalemate. Somewhere, over the horizon, various enquiries wander the landscape like medieval armies, and no one knows which side they will support when – if – they finally arrive.