The honours go to you JW. They are storm holding tanks for the large sewage pumping station a little further up the road.
Maidstone and the surrounding area has a Combined Sewer system. This means that the sewers take the rain water as well as the foul stuff. In conditions of heavy rain, the pumping stations and sewage works are unable to cope with the sheer volume of water trying to get through the system. The wet well at Allington Water Pump Station spills over into those tanks and when conditions calm down, the water is drained back into the wet well, from where its pumped to Aylesford Sewage Works (which doesn't have storm tanks).
Pretty much all the sewage from Maidstone and the surrounding area goes through Allington, so it's a substantial pumping station.
Motney Hill and most of the others I listed earlier have storm tanks on site.
If those tanks are unable to cope (ie if they fill up) they spill through a screen into the Medway, upstream from Allington Locks (hence non-tidal).
Over to you JW.