According to the BP RNS - "Gas supply price will be linked to Italy's "Heren PSV day ahead mid" price assessment". That market seems less transparent though, and I think one of the CPRs said that this trades at a small premium to TTF. I'm sticking with using TTF and multiplying by 0.010764 to get to β¬/scm.
Assuming they haven't already done so (lets not rule that out given all the sell-on-any-news since Feb), Β£500,000 would take about a week on current traded volume.