I think it varies tbh. I can remember some corkers at completely different times of the day/night - 7.30pm (I was on the roof of the greenhouse, never got down so fast from anywhere), 9.30am (I was judging at a dog show in Lincs that day and suspended judging, the lightning was rather worrying - that was probably an MCS), 2.30pm (sounded like the guns on the battle area nr Cressingham, it was actually constant thunder); 11am (remembered for all the wrong reasons i.e. leaving a velux window wide open whilst out shopping and finding a junior water fall...). These examples were all in the 80s/90s. Since then the better examples have all been post midnight with very few daytime storms bar the occasional popcorn variety.