I would say you are def in the right and with the invoice stating 'plain trim and cushons' you have the proof.
My advice is to not take the cushon's offered if they are sent out to you because it might not legally look good further down the line. If you except them you are almost saying 'that is exceptable I'm happy now' which is not the case therefore, you have to wait out for the whole thing to be fixed or your money back. Send away any delivery man with the cushons, and tell family and neighbours to do the same.
I think the best think to do would be to get on the phone or internet and get free advice from citizens advice, or consumer direct. Citizens will probably recommend consumer direct anyway so try them first they speciallise in helping with this problem. You can go online with them and fill out a form telling them all the details like company, when bought, for how much etc. And they will contact you back or you can ring them for one to one.
Good luck, your in the right and have the proof.
P.s Carefully take photocopies of the invoice so you don;t lose it. And if the sofa in question is on the website, print a picture off with info, and take a photo of your sofa for comparision.
PPs Just I thought did you see the sofa was wrong when they delivered it? And did you still sign for it? Does the delivery note state any small print regards faulty and incorrect products and signing for them the smalll print often states its your job to check before you sign. I still think your in the right, but I think you need to get your head round these facts as they way effect your claim slightly?