Oh my actual god!!! I've just read both of these threads. I'm astounded at the people who are saying you're unkind and you and your husband are not very nice!
So let me get this clear -
You requested RSVP's by a certain date so you could provide (what sound brilliant) personalised party bags for those you know were attending, they were 12 pounds a pop.
You, I imagine, organised pizza for the kids and I have no doubt you asked for dietary requirements to be made clear to you.
You organised what sounds like an amazing party with a magician (all kids (and me) LOVE a magician.
You decided to do this indoors so the children were warm, comfortable and safe instead of taking them in below freezing temps to a play park.
A person who had not replied to the invite turns up, essentially at this stage uninvited, informs you of dietary requirements of her child, you accommodate, child and parent eat pizza and take leftovers home and you also apologise to say you were sorry that due to the non response you didn't have a personalised party bag like the other kids got. (At this stage you would have been expected to also pay £12 for party bags for those who didn't rsvp 
on the off chance they turn up ! Dear god! I don't know who has cash like that to throw away!)
Amanda contacts you to say her son felt excluded and you replied with a very sensible solution and gently say, it's ok we all make mistakes or forget things at times but here's how you can resolve it for your son. I would have basically said here's number of magician, if you had told me he was coming he WOULD have had a bag, but you didn't, you'll need to be honest and explain that sometimes in life if we are disorganised and miss things and we sometimes don't get the desired outcome and this is a perfect example of you not being organised and I'm sorry that because of this James has missed out.
She tries to call you and you don't answer (I rarely answer my phone, tbh it rarely rings) as you are at a conference.
Neil the husband is asked a question regarding your phone and gives her the honest answer and leaves.
Where the hell have you done ANYTHING wrong?!?
It is the party just responsibility to cater for lazy or disorganised parents.
No! It's notX it's your responsibility as a parent and adult to make sure you are organised for your child to live their best life, this includes rsvp - ing to party invites to ensure your child is catered for, this includes knowing your child's timetable of things like what's needed for clubs etc. when pe kit is needed so they aren't wearing the lost kit stuff and being made fun of by other kids.
I'm aghast at all these mumsnetters that don't see how unhelpful it is for moulding their children to understand that sometimes in life there will be disappointments, how we avoid these disappointments is learning to be accountable, learning to accept that things don't always go your way etc. and also how to deal with the emotions these things bring.
There IS a life lesson here and it's not for James, it's for Amanda, be better organised, have more respect for parents who are doing something nice for your child, have more respect that they are forking out cash to entertain and feed (you ) your child.
I'm honestly ragin for you - mine is 21 now and I absolutely wouldn't have put up with any of this pish, if I did something that affected my sons happiness or something that disappointed him I'd apologise to him and say I'm sorry , this is what happened, it's my fault but what I have learned is to make sure I respect you and other children/parents by responding to them as requested!!!
Luckily this isn't in Scotland in my sons formative years, Amanda would have been to foxtrot Oscar and told to make up for her own mistake!
You did great op and I would also like a party bag!!!