what's done is done, the guy does not need reminding, even if it was a rip off and it is impossible to say that as no one has a clue as to the amount of handwork involved. Anyone quoting had no idea what he was facing in that field of rubbish 3 feet high etc.
I bet there is no one here ( in their right mind ) would take a job on like that cheaply. How many tales on the machinery threads of people wrecking their kit on a large lump of concrete or steel unexpectedly found in a ploughed field yet alone a mini jungle. just one stake running through a large tyre can cost the best part of thousand pounds forgetting the time lost!
Remember as a child a local contractor took on a job for us, ploughing a previously uncultivated field ( certainly since the Victorians ) It was on a very steep bank. He had a very large Cat and 4 furrow stump jumping plough. Sadly it was not up to the job and with the weight of the crawler behind it the plough hit rock and disintegrated on its first pass
As said we do these kind of jobs. Basically we are £600/day all in to do it but by actually walking the site properly you can get an idea of what its all about. If you don't walk it or even look at it then you have either massively over charged or its going to cost you in damage.
However never have I ever not been able to give an idea of total cost due to the timescale so the OP's family have paid well over the odds for this but I suspect its a landscaping company and not a farm contractor. On jobs like this we want to make 35% profit otherwise its not worth the hassle factor but then you get some nice little add ons like having the wood etc. But £5000 from the pictures is just silly.