Accepting

• Thank you for the invitation - we would love to come. Two of us, and we'll be there for [time].
• Delighted to accept. Count me in for the [date], and let me know if you need a hand with anything.
• [Name] and I are thrilled to be asked. We'll be there, and we're already looking forward to it.
• Yes please! I'll be coming on my own, and I'm happy with either menu option.

Declining

• Thank you so much for thinking of us. Sadly we're away that weekend, so we'll have to miss it - we hope the day is wonderful.
• I'm sorry, I can't make the [date]. It means a lot to be asked, and I'll be thinking of you.
• We'd love to have come, but we're not able to travel that far this year. Please pass on our very best.
• Unfortunately work has me away that week. Have a brilliant time, and let's find a date afterwards.

You never owe a detailed reason. One short line is enough.

Not sure yet

• Thank you for the invitation. I'm waiting to hear about [shifts/childcare] - can I confirm by [date]?
• We'd really like to come. Is a reply by [date] still in time for your numbers?

Give a date you will definitely answer by, then answer by it.

Replying late

• I'm so sorry for the slow reply - the invitation slipped past me. We'd love to come, if there's still room.
• Apologies for leaving this so late. Sadly we can't make it, and I'm sorry not to have said sooner.