I think you may have answered your own question;
"on the house inventory the vendors ticked washing machine - which we didnt query"
The Inventory that formed part of the Contract of Sale said "Washing Machine" and that is what you got, so unfortunately, it doesn't look like you have much comeback other than to put it down to experience.
There is nothing wrong with amending or detailing the inventory. Eg if there is no tick box for washer-dryer, tick the washing machine and write underneath "washer-dryer" or the model no. etc.
The last time I moved, I agreed to purchase the cooker - but when I finally moved in - (8 months later because of the pain-in-the-@rses buying my house) - it had been replaced by some crappy, old-fashioned upright thing.
I didn't do anything about it, though, because
1.) they had been rather patient waiting 8 months for me to complete the sale, and
2.) there was b�gger all I could have done about it if I had wanted to - since the inventory said "cooker" and that was what I got.