Aha - that led me to https://bmrs.elexon.co.uk/system-prices https://bmrs.elexon.co.uk/system-prices , which shows that for the last week prices have been hover

Aha - that led me to https://bmrs.elexon.co.uk/system-prices https://bmrs.elexon.co.uk/system-prices , which shows that for the last week prices have been hovering in 80-180 range, and there was only one period of negative pricing during the day.