This is the response I have just sent, courtesy of Sarah and then Pictish:
"Dear F,
I am sorry if I wasn't clear before. I was hoping that you might realise just how rude you are being, without me having to spell it out to you.
You simply cannot expect to send someone a save the date card, fail to invite them to your wedding, and then request them to arrange childcare and put in time and effort to do you a wedding related favour!! The kind of thing you are asking of me is only really appropriate to ask of a guest (and even then only if it's not putting them at an inconvenience) or a professional decorator/planner.
I understand that this is your wedding, and you have every right to invite whoever you wish to attend. However, I am frankly astonished that you cannot appreciate just how rude you have been, in asking me to help you decorate the venue for a wedding you chose not to invite me to, for the benefit of those who you DO regard well enough to invite!
So just to be clear in order that there is no further ambiguity, I wish you nothing but the best for your wedding and marriage, but I regret I will not be able to help you.
Best wishes,
Tidy"