The brother of my best friend of 30 odd years is marrying on Saturday and best pal is to be a witness. My pal's DP hates weddings and so, a few weeks ago, he asked me to go in her place and with her blessing. I was delighted to accept and have arranged for someone to look after the DC so I can spend tomorrow night at pal's and go to the wedding with him the following morning.
Of course I was under the impression that S has asked the groom if he might take me instead of his DP.
No.
I called best pal tonight to discover that the bugger, "thought it would be nice if it was a surprise!". 
I'm SO embarrassed. I can't back out now (can I? CAN I???). I am (was!) looking forward to catching up with best pal and his DP tomorrow and having the first day and night in 16 years in their company without the DC. I flustered and blustered something about "You can't do that! S! FFS! You just CAN'T!" and my pal laughed and said alright, alright, I'll tell him.
They're a laid-back family, it's not a posh society wedding and they know me and my friendship with pal of old and all are very comfortable with it. X's main reason for not telling was to surprise his Mum and Dad (who I view as my second parents) which is sweet, BUT... !
Oh no, feck me sideways, what am I going to DO?!
My pal is now going to call his parents tomorrow to arrange for me to be able to drop my travelling bag at their house on Saturday morning as it's far nearer the reception than his own home and would make it easier for me to collect it and get home later that afternoon, so his parents will be told and hopefully not mortally offended. I don't think I will offend them per se, they've always treated me as family, but fuck only knows what the groom (and bride) will think. I can't just roll up univited when they're expecting to see pal's DP and being told the day before they wed that best pal is bringing me instead is just not on!
HELP!