How to sow delphinium seeds successfully?

I bought some Delphinium pacific giants mixed seeds from Wilko last year. Sowed two batches in Apr and May this year according to the instructions on the packet (into seed tray, cover with a thin layer of compost), but none germinated.... Wondering if there is any trick in doing this please? I was reading on some website (https://www.gardeningknowhow.com/ornamental/flowers/delphinium/delphinium-seed-planting.htm) saying that they need a cold start so should be in the fridge for a week? And when is the next window for me to sow these pls?

Thanks for any comments / advice!