I didn't read it until the following day but they have sent an email this morning claiming I haven't responded and it's been over a week, no doubt they are charging my ex for the letters they are sending which is what I thought was illegal? I would also think that being it was a public holiday that day cannot be included when they say I have had over a week to respond.
1. Public holidays are immaterial in correspondence from a lawyer because correspondence from a lawyer is not binding, meaning you don't have to reply within the time frame they give you, and in fact, you don't have to reply at all if you don't want, and there's not a thing they can do about that.
2. What your ex's lawyer chooses to charge your ex, is between your ex and your ex's lawyer. Don't worry your little head about whether or not they're allowed to charge for letters sent on public holidays. That's your ex's dispute to take up with them if she wants, not yours.
Why would it be illegal to charge a client for a letter sent on a public holiday? Lots of lawyers work outside normal business hours, which means that lots of lawyers send correspondence outside normal business hours. It's normal.
I assume there are more important issues at play than what day of the week correspondence is sent on. Focus on the big issues, not the completely irrelevant ones.