The following phrases are useful for formal written invitations.

· Inviting/Offering

We should like to invite you…

We should be very pleased if you could…

We would be delighted if you could…

· Thanking

Thank you (very much) for your (kind) invitation to…

It was very kind of you to invite me to…

I was delighted to receive your invitation to…

· Accepting

I would be very pleased to …

I should be delighted to…

· Declining

Unfortunately, due to… I am unable to…