Agreed. Very common practise on the continental waterways where things are just so much bigger. When cruising European waterways when my boys were young their job was always to pop up front with their blow horn and let me know if there was anything approaching whenever rounding a bend. Should be the same over here. I still hoot on blind bends.Don't col' regs apply on the Thames as on the sea?
e.g.
Surely if a mobo of any type approaches a blind corner or pulls out where viz from the mobos bridge is way back near the stern, then the skipper of that mobo has the responsibility of a) putting a look-out at the bow where there is a communications system set up and b) sounding the 'ships siren/horn' to alert those in close proximity around the blind corner or bridge?
In the instance of the coxed quad hitting the barge... I'd have thought this was as serious a misjudgement by someone as it can get. I would have thought it should at least be dealt with extremely seriously by the barge company and by the safety officer at the Rowing Club/School.
Very sad for those youngsters involved who from what you say may be just 12-15yrs of age.