I think it may be a mixture of both.
When they filled in the seller's info forms they said there were no deadlines. They also originally were tying a purchase in as well, but decided to sneak one in. They're now buying a new build and the developer want it to complete before their financial year end otherwise the deal they've been offered won't stand (allegedly).
The solicitors they used when they bought this house were crap so they didn't have some of the information they should have. Our solicitors asked them for a copy of the original planning permission for the site but they never provided this. Our solicitors had to get it from the local authority themselves.
There has also been an issue with the roads being adopted on the estate so our solicitors asked them to buy an indemnity for us so we would not have to pay for road maintenance to make them adoptable (because their original agreement with the developer is not transferable in a sale). They dragged their feet over this and just wouldn't do it. They just couldn't understand that it is their problem and any buyer will expect this.
There was also an issue with management fees where they refused to accept to they will have to take responsibility for the management fees up until completion. We shouldn't have to pay the full year when they lived there for half of it. So they dragged their feet and are clearly resentful at having had to pay this. Bear in mind that they have no EA fees to pay, so it's particularly silly.
Then this week they suddenly decided that they would cough up and supplied all the paperwork to our solicitors. Within 5 minutes of their solicitor emailing that through they phoned to say they wanted to exchange there and then. Our solicitors hadn't even had a chance to look at it never mind do anything with it. We haven't had a contract to sign at all up until today because the vendors hadn't supplied the information to produce one.
So now they're being total arses and going on about how they have to complete on the 24th and must exchange tomorrow (Monday is too late). And it appears that this is at least in part because they've prematurely booked movers. At no point have they given our solicitors a deadline - and certainly they couldn't have done since they hadn't answered the bloody queries.
DH had to speak to an irate Mr vendor tonight at dinner time (and he came so close to telling him to fuck off and that we weren't going to buy the house at all in anger 
). The solicitors sent through our paperwork for us to look at and we just need to go in and sign it. We can do this tomorrow so hopefully the solicitor will make herself available for this.
On top of all this stress I'm really pissed off at more than. I have already sorted the buildings and contents insurance but they haven't sent me the documentation. They've taken payment and sent me an email so I could theoretically log in and download it myself but no. Apparently the account is suspended and I need to contact a customer services manager. I spent 30 minutes on the phone to the wrong department (because the website gave me the phone number that takes you through to car insurance) and the (very nice and helpful) guy in the phone couldn't get me through to home insurance nor could he get into my account. So he's put a note on my file and I need to phone at 8am tomorrow. I also tried the web chat but the guy on there claimed that he couldn't find the account and said I'd need to phone. I do not need this stress, especially when I know I've sorted it (and I can see they've been happy to take my money).