Hi,
If the pump won't stop after you have stopped the flow, then the pressure switch is suspect. The amps load across the points in the switch usually fuse/melt/weld together over time. The amperage is quite high on 12 direct current motors. However it may not be cutting off as quickly as it normally would due the the 1/2" hose you are using. It could be that this hose is quite flexible and is swelling under pressure. I would try the pump with a short outlet hose first.
Next, I would carefully remove the pressure switch housing and try to prise the microswitch out. You will be able to operate it manually and you will hear the contacts clicking open and closed. If not then odds are the pressure switch has failed. You may be able to get a cheap microswitch replacement from Maplins or some other electronic supplier by getting them to match up the sample. Also check to make sure you won't need terminals as well.
When removing the housing make sure you don't loose any tiny little spring/s and/or plunger.
If you can't get a cheap microswitch, I would buy a decent controller with electronic deadend and operate the pump using with this. I would say that the new Spring 'analogue' controller that Gardiners and other's have is a good purchase.
Spruce
I have been offered a varistream in a couple of week...fingers crossed. Am i right in thinking tht a varistream will cut out water supply when I stop the flow?
A varistream will yes. It will just need to be set up. It looks like the Aquatec pump draws very little current when compared to the Shurflo the Varistream is designed for so may have to be set very low on the Varistream to suit the Aquatec. ( I don't trust the specs of Aquatec website for that pump, although they could be giving current draw for the 230v unit.)
Williamson pumps, who make the Varistream. don't list other makes the Varistream is compatable with, so this is going to be by trail and error.
Edited: If it is compatable with your pump, then yes you will be able to use a tap or Aqua-dapter. We use Aqua-dapters on each van, one has an old analogue Varistream and the other 2 have digital Varistreams - all work fine, but all our pumps are Shurflo's. By enlarging your photo I see the your pump delivers 1.3GPM which is similar to the 5.8lpm the Shurflo's deliver. So I would guess the amp draw will be similar.
Spruce