Thank you so much for the invitation, but I'm afraid I have a prior engagement.
Thanks for the invitation! I'm really sorry, I can't come - I've already got other plans.
I'd love to, but I'm afraid I can't. Would another time be possible?
I'm very sorry, but I won't be able to make it. Another time would be great, though!
It's a lovely idea, but I won't be able to join you this time.
That's very kind of you, but …
Could I join you later? I just have to finish something.
Today's not so good - how about tomorrow?
I understand what you're getting at, but (I'm just not interested/I just can't /it just won't be possible) at the moment.
Look, Michael, I'm sorry, but it's just not going to be possible at the moment.
I'd like to help you but I can't take on any new commitments at the moment, I'm afraid.
Actually, I think this is something you could do really well yourself!
Why not have a go yourself, and give me a shout if you need help?
I need to think it over. Can I get back to you later?
I'll think about it and let you know, OK?
I'm afraid I'm not familiar with that, but have you asked (X)?
Sorry, but that's not really something I can do. Maybe (X) can help you.
Oh dear, I'm terrible at that sort of thing! What about getting (X) to have a look at it?