New render cracking immediately

I'm afraid it sounds like a failure in material possibly exasperated by heat. Strong cement based renders are prone to cracking and if soft sand was used this would also shrink because of the presence of clay. A suitable render mix would be one part cement one part lime to 6 parts clean washed fine sharp sand.

To me a good traditional rendering job always depends on the weather conditions. Also the sand to cement ratio & most importantly of all you MUST use a sharp washed sand! Also now known as plastering sand. Basically you don’t want to apply the top coat/ finishing coat in direct sunshine as this bakes/drys out the mix on the wall to quickly & the render will be prone to cracking after you’ve sponged finished it. Also the scratch coat should always be 4:1 & top coat 5:1 ratio. Don’t forget to add a waterproofer & plasticiser to the water too. The most common reason for cracking though is that a soft sand has been used. A building sand for laying bricks basically. It shrinks & isn’t strong enough to bind together. (Not gritty). Some people use it because it’s creamier & easier to work with but it’s totally the wrong choice!! It will always crack eventually.

So this could be down to a few issues. One being they didn't leave enough time before scratch coat and topping, so now scratch coat had shrunk hence the cracking. Or they didn't prep the wall well enough before topping so it's dried to rapidly again leading to cracking. It needs to come off and be redone properly. I would recommend a modern monocouche render (K-rend) perhaps slightly more expensive but with a fibre glass mesh tied in it doesn't crack, it's also pre coloured to a colour of your choosing, so eliminates ever having to paint.

it could be down to the fact no waterproofing was used allowing the scrafch coat to cure to quickly

You need to get them back to look at it and give them the opportunity to correct the problem. If they are unwilling you will need to think about a different approach. (Legal)