This just keeps getting better...
Looked on the LOT website at the fare breakdown.
LOT tells me I paid £482 for the ticket, including £138.60 in YQ. BMI told me I was paying £213.20 in YQ
I believe BMI charged me in the region of £560 (not showing on the credit card yet), so the difference between the 2 YQ prices makes £482 up to £560 or so.
Is there a legal obligation to provide an accurate receipt for a transaction? Who do I speak to next?