the part that has broken has no form of protector to help deflect stones kicked up from the front wheels
Not sure I'd expect frame protection on a chainstay, certainly not from rock strikes off of the front wheel. That looks to me like a side-ish impact, so a rock flicked up from the front wheel and curved round the crank, pedals, and your feet (or bounced off of another rock perhaps) before hitting the bike mid chainstay with enough momentum to cause that kind of damage? You should buy a lotto ticket!
What does the inside of the chainstay look like?
In all fairness, regardless of cause I'd hope for a goodwill gesture of some kind. Is the £270 cost price?