There is a lot of misunderstanding in this discussion on Ashwell and Morden.
The station is on the Chilterns which is a chalk escarpment. This means that its water supply is from local boreholes into the chalk aquifer below, not from a reservoir. This applies all the way along the Chilterns, including the Cambridge area.
Those boreholes were drilled by small local companies and the legacy of that survives to this day. At privatisation the water at Ashwell and Morden was provided by Lee Valley Water Company, which is now part of Affinity Water. Cambridge had its own water company, which is now part of South Staffordshire Water Company. There is no existing network of water mains, like those operated by the big water companies, because the water comes out of the ground from under where people live, not from a reservoir far away.
This system has worked well for hundreds of years, but there is severe water stress now because of two developments, leading to over abstraction of the chalk aquifers in recent years. One is population growth: there was enough water in the aquifer to supply a city of 100k people, but Cambridge is now 150k and still growing. The surrounding areas are growing rapidly too. The other is climate change: summers are hotter and drier which reduces water supply into the aquifers. Hot dry summers also result in agriculture abstracting more water for irrigation. Agricultural water use is very significant: 10% of all water use in the East of England is agriculture, the next highest region is at 3%.
As well as the economic implications, over abstraction from the aquifers has severe environmental implications too. At the edge of the chalk the water comes out of the aquifers in springs, which feed almost all of the tributaries of the River Cam. These chalk streams are a globally rare habitat, there are only just over 200 of them and more than 20 are in Cambridgeshire, about 10% of the global total. Maintaining water flow in the chalk streams is critical for the rare species that live there, and it also matters for the global perception of Cambridge because the River Cam has to have enough water for punting. The springs at Ashwell are the source of the River Rhee, one of the biggest contributors to water flow in the River Cam at Cambridge.
It has taken a while to get governments to realise that water scarcity is the most important constraint on population and economic growth in Cambridge, but now things are happening. There will be a new pipeline from Grafham Water, near Huntingdon, and a new pipeline from Lincolnshire. The planning process for the Reservoir in the Fens is proceeding on a similar timetable to East West Rail. Although only indirectly, the Grand Union Canal Transfer will help too.
Like all big infrastructure projects, they take time to design and build, the Cambridge area is going to continue to be water stressed for at least 10 more years. That's why the priority now is Tempsford. And even when all that new water infrastructure is done, it isn't going to be delivering more water to Ashwell and Morden. The best Ashwell and Morden can hope for is being able to do more abstraction from the chalk aquifer because Cambridge is doing less.
But desalination won't be happening. If it was a magic solution then it would be happening now, instead of pipelines from Grafham and Lincolnshire. Saline water is (a) at sea level and (b) at the seaside. Ashwell and Morden are neither, the station is about 90m above sea level and 50 miles from the seaside. Desalination is also energy intensive, while there are sources of rainwater that can be piped and pumped over similar distances, it won't be competitive.
Ashwell and Morden is tricky politically because the station is in Cambridgeshire but Ashwell is in Hertfordshire.
Ashwell and Morden station is in Odsey. Although Ashwell is in Hertfordshire, Guilden Morden, Steeple Morden and Odsey are all in Cambridgeshire. Furthermore, Guilden Morden, Steeple Morden and Odsey are likely to be within the area of the Greater Cambridge Development Corporation. It is possible that the Greater Cambridge Development Corporation will look at Ashwell and Morden station again, but being in the Affinity Water area is likely to be the main administrative constraint.