Some of the people replying to this and suggesting you can pressure your vendors probably don't have experience of what the market is like right now.
It's brutal. There is nothing on the market, particularly as you move up the chain.
We are in the same position as your sellers.
We put our house up for sale because we saw a property we liked, but the seller wouldn't accept our (asking price) offer until we were under offer. In the 1 week it took for our house to go on the market and under offer, we lost the house we liked to another buyer. We have seen every other house on the market within our budget and area range, made a couple of offers, but still haven't managed to get an offer accepted.
Our buyers asked if we would consider going into rented at the time of making their offer, we said absolutely not. We're not selling our house and uprooting our family until we have found something to move into and refuse to make a forced decision on our house purchase.
If we lose our buyers it's mildly annoying but not a big deal as we'll find other ones within a week. That's the nature of the market. The money we'd lose finding new buyers, even ones who offered less, would be less than we'd lose making a bad onward purchase. We have been completely upfront with our buyers about this, so don't even feel bad about making them wait - if they can't wait it's for them to judge and they are free to pull out.
So just be a bit careful if it's a house you really want.... Back in 2014 I waited 9 months for a chain to complete because my sellers were looking and I really wanted a particular house. I think the market is much much harder now and for sure a complete seller's market.
That said, has your seller been actively looking? We've been letting our buyer know that we are doing 4+ viewings a week. If you've not heard anything from your sellers maybe they're getting cold feet or not serious.