Throughout the movie the amount of blood and dirt on Jenny and Steve can change instantly. An example is when Jenny and Steve are hiding underneath the wooden hut (in the lake.) In one shot Steve's chin dips into the lake and this causes his chin to become mostly clean from dried blood and dirt. But in the next shot all the dried blood and dirt is suddenly back covering his entire face.
During close-ups of Steve's hand tied up by barbed wires we can clearly see that his hand is soaked with blood. However, in the immediate scene after, where he managed to wriggle out of the wires and bring his hands in front of him his hand is only bleeding a little bit. There was no time for him to wipe all the blood clean.
The Satnav, when the couple are travelling to quarry, shows they are going to Slapton Quarry which is in Devon in the South West of England whereas the family appear to all come from the South East of England.
After Steve crashes his car into the tree, resulting in him being pinned between his seat and the tree branch coming through his side window, he could have reclined his seat and exited the vehicle through the rear passenger side door.
It is likely in his panic that he did not consider this option, but it appears this is how the gang extracted him from the vehicle.
When Bonnie the Rottweiler sneezes over Jenny (during the beach scene), Brett the animal's owner, shouts "come here girl". Either he's thicker than thick or the Rottweiler has an identity crisis because it is clearly a boy dog.