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In my garden pond I saw a bundle of frogs seemingly smothering one reddish coloured frog. Being British and always favouring the underfrog I sprayed the "attackers" with the hose until they all went.
The victim frog was indeed reddish in colour and seemingly very weak. What is going on? Is the red frog not well? Why are the other frogs attacking it?
What you witnessed was almost certainly mating. As this article from TheScientist explains,
“Explosive breeding” is a common reproductive strategy among frog species. Males congregate near sources of fresh water and scramble frantically for any females passing by. Only about 5 to 10 percent of males will mate successfully, estimates conservation biologist Marc Sztatecsny, a lecturer at the University of Vienna. “There’s so much competition it makes sense to try to hold what you have,” says Sztatecsny. “The males will clasp anything. They won’t let go.”
The drawback is that females often die in the mating melee, apparently losing their chances to reproduce.
It's likely that the frogs in question were Rana temporaria, the European common frog. This would fit with the colouration you observed. Wikipedia notes:
During the mating season males' throats often turn white, and their overall colour is generally light and greyish, whereas the female is browner, or even red.
There's some great footage on YouTube of common frogs mass breeding in the Val d'Aveto, Italy. And here's a photo (source: Wikipedia) showing the striking colour difference between males and females during breeding season:
