I'm baffled by this one this week. I've managed to work out that the town is Durobrivae, athough it seems it was also called Durobrisin and Durobrivas in the past.
What I can't work out is the feat of 15th century engineering. There's nothing marked on my 1:100000 road atlas of relevance NE of Whittlesey.
It's got to be something staring me in the face. Help!
A hint to start . It was built by Bishop of Ely ( and later Archbishop of Canterbury) John Morton in 1490.
Happy to give you the answer later if this is not enough.
Morton's Leam -
Morton's Leam was built in the last decade of the fifteenth century. It served as a prototype for later schemes across the region, by channelling the Nene's unruly course direct from its entrance into the fens straight to an outfall into the sea.
Sorry if I seemed to be panicking but I normally enter the comp online and the website was down over the weekend. I've sent my answer off by snail-mail now.