Hmm... tense.

I just realised that for the Steve letter I'm going to have to change it to past tense, since it's over.
I'll try this...

Dearest friends of the [name] Society,

Firstly, I’m very sorry for my late reply, I’ve been travelling around on business and upon my arrival home was pleasantly surprised to have received your invitation to the annual Garden Party. I am sorry that I was unable to attend this gathering but I was out of the country at a kite-flying convention that day. I hope you will accept my deepest apologies for my absence and I hope that everything went well and you all had a marvellous time.

Yours Sincerely,
Steve

I think that works...