The couple had been courting since they met at Aberystwyth University, in Cardiganshire in 2001, family friend Dafydd Dafis said.
John's move to a teaching job in Caernarfon, and Miss William's continuing her life in Cardiff, are also said to have put a strain on the relationship.
Council House has not commented on the split. There had been speculation about a possible engagement in recent months.
The woman next door, Megan Edwards, said: "It's a surprise, because it had seemed very stable and very steady."
She said she thought John's move to Ysgol Huw Owen in Caernarfonshire had "a lot to do with the final stages" of the break-up.
The bloke in no 13 reported that the couple had seen each other no more than once a week since then.
"One must suspect, though I don't know that this is absolutely certain, that it is more his decision than hers," said Megan Edwards. "He has said in the past that he is too young to get married. But I am led to believe that there is no-one else involved and that it is a decision they have come to mutually."
Will in no 9 said the pair had reached an "amicable agreement" to part.
Miss Williams attended John's graduation ceremony in Aberystwyth last year. It was the first time she had attended such a high-profile event, at which Mrs Jones and other senior Joneses were present.
Miss Williams left her family home in Cardiff on Saturday morning without speaking to the assembled neighbours.
She works as a till operator in Tesco's St Mellons.
I know it's a boring post. Just as boring as that other boy splits with girlfriend story that is splashed all over the news. Yawn!