Having had a batch of spawn as the first evidence of frog activity I was also intrigued by JoeB's query about who makes the first move. I suppose intuitively, since the males allegedly croak to attract females, then as Posie says they most probably arrive first. There is a nice BBC item on the subject
BBC - Science & Nature - Wildfacts - Common frog, grass frog that confirms Posie's suggestion.
Another batch of spawn overnight, lost count now, but I have noticed there is one batch that was laid in the original tranche 27/28th Feb that is not developing. Because of the position I think it more likely that the reason for this is that they were not covered by the male rather than (as some others) they were affected by the freezing. So, was probably too cold for him to come out to play
M