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Labour's Commons debate this week on awas designed to embarrass the Liberal Democrats but it induced a red face or two among Plaid Cymru MPs as well.

Plaid's parliamentary leader, Elfyn Llwyd, voted in favour of a new tax on properties worth more than £2m; his colleagues Hywel Williams and Jonathan Edwards didn't.

Mr Edwards had told the debate: "We are minded to support wealth taxes, and we therefore welcome the motion, but it is a bit thin on detail. Can the shadow minister reassure me that farmers will not be dragged into the new tax because of the value of their land, and not necessarily because of the value of their property?"

The shadow minister, Labour Treasury spokesman Chris Leslie, was grateful for his "support".

"Obviously there is a difference between residential and corporate arrangements, but our motion says that we want the Treasury to bring forward proposals at the earliest possible opportunity.

"We have seen the proposition set out by the Liberal Democrats and used it as the basis for our motion, but let us see what further options can be drawn together. We think that it would be a good idea, for example, for the Chancellor to commission the Office for Budget Responsibility to present detailed suggestions of ways in which the arrangements might work."

So the detail remains thin. The suggestion is that owners of £2m homes would pay a one per cent annual levy on them - £20,000 a year. But we don't know whether Labour's mansion tax (or the Lib Dem version) would be UK-wide, whether it would be administered by the Treasury or through a change in local government finance - or how property would be valued.

But that didn't prevent Labour exploiting the Plaid Cymru confusion. Shadow Welsh Secretary Owen Smith took to twitter to wind them up: "I see some Plaid Cymru MPs failed to support our #mansiontax vote yesterday, preferring to back multi-millionaire landowners."

Cue this response from Plaid Cymru Treasury spokesman Jonathan Edwards: "We support wealth taxes, disturbed that Labour wont exempt the traditional Welsh family farm!"

Owen Smith queried the arithmetic: "How many acres can you get for £2m? About 300 by my reckoning. And the traditional Welsh farm? About 90."

Jonathan Edwards: "So farmers wont be exempted?"

Owen Smith: "Not ones whose houses are worth £2 million. The clue is in the name, see: #mansiontax."

That appeared to generate an outbreak of consensus. Jonathan Edwards: "If Chris Leslie had said productive agricultural land was exempt we would have supported. I did ask very kindly."

He explained later: "All they had to say was that they would exempt farmers who actively work their land. If Leslie had given that guarantee when I intervened, I would have been through the lobby faster than Owen."

Just to clear up any confusion, I asked Elfyn Llwyd where Plaid Cymru stand on a mansion tax. "The three of us voted in different directions the other day....I voted on the basis that I understood that it would not apply to farms. My colleagues, however, had a different understanding.

"They thought it would apply so therefore they were against and I went in favour, but the point is, if I find out in any shape or form it affects farms then clearly it's a non-runner; if it's purely to do with properties, in other words domestic property you live in, then that's a different issue altogether."

So if farmers are exempt from any mansion tax (if it ever happens), and you're lucky enough to live in a home worth £2m a year, it might be worth buying a few chickens for the garden and telling HMRC you're a farmer - it could save you £20,000 a year.