When you complete the various questionnaires for the solicitors, you do sign to say that all rubbish, furniture, etc. will be removed from the property. I know this because we literally completed the forms for the sale of our own property yesterday. However, you do NOT agree to leaving the property in clean condition, and as far as the windows are concerned, or any other problems with the fabric of the house, I'm afraid you're on your own OP, as the onus is on you to check these things. If your surveyor could have accessed the window, and keys were available, then you might have a claim against them, but if you read the survey report that you had done, it may mention that the window hinges were broken or not working properly, and need attention, or if the surveyor couldn't reach them at the time, because for example there was a piece of furniture in the way, or packed boxes, basically anything prohibiting easy access to the window, then they would have stated that, and are under no obligation to check. It's a bit like when there are carpets in the rooms, you might expect the surveyor to have checked the floorboards or whatever underneath, but you will always find that they state 'floors in the room were not checked as fitted carpet was laid'. You'll find that they don't move ANYTHING to check underneath or behind it, so for example if there is mould on the wall, and the vendor has placed a cupboard in front to hide it, the first you will know about it, is when you move into the empty property. As a previous poster said, I'm afraid those things fall under the title of 'Caveat Emptor' (buyer beware!)
Regarding the rubbish, it really doesn't look that bad OP, and it will likely cause you a lot of stress and heartache if you try to sue the vendor, in order to get them to pay for getting it removed. If they happen to have left a forwarding address you could try contacting them and telling them that unless they clear it within 24 hours, you'll be instructing someone to do it, and sending them the bill, but it's unlikely they'll respond, unless there is a LOT more rubbish than you've shown in the pics.
I'm sorry to say this OP, but as a first time buyer, we all tend to be a little naive about what to expect when we move in, so my advice is, get shot of the rubbish yourself, check the survey report, and contact your surveyor if they haven't mention checking the window, or not being able to check the window, to see what they say, but then put it down to experience. We're all older and wiser the second time around.
You were kind in allowing the vendors to have the extra time to move their stuff OP, but on moving day you do have to allow for problems to occur, for example, it takes time to pack the contents of a house into a moving van. I'm assuming the vendors moved themselves, and they probably didn't allow themselves enough time to do the job. It's also possible, as happened to us on one occasion, that the van has had a flat tyre, which they've had to get changed, or they've had to stop and fill it with fuel, or the van has broken down and they've had to wait for a replacement. Maybe they overslept, maybe a child was sick. All of these things do happen, so next time you move, to avoid a lot of stress, it's better to anticipate not being able to gain access until late in the day. Also, you won't be at the bottom of the chain next time, and just so you understand how it all works, on moving day the solicitors start at the bottom, passing over the money from the beginning of the chain, quite often a first time buyer, to the solicitor of the vendor at the bottom of the chain. The vendor's solicitor then has to check that the money has arrived at their bank, and arrange for it to be forwarded to the next property, and so it goes on. So if for example there were 6 people involved in the chain, and each transaction takes an hour, if you're the last person at the top of the chain, it's highly likely that you won't complete until 3pm or possibly even later.
Hope that's helped to put things in perspective and explain some of what you've encountered today. Have a good night's sleep, and trust me, it will all look better int the morning. Wishing you every happiness in your new home.