I understand regarding the issue on your mains feed for the cottages, so will focus on that.
There are a few tests which can be done to look at if its a supply pipe or pressure issue, my guess it ma be both.
Take the standing pressure from the mains, with no demand (taps running) in any of the properties, you can picking pressure testers on screw fix here, they just screw on to a garden tap.
Secondly I would do the same but turn one tap on, check the pressure, two taps on, then three, and look to see what the pressure does, I would expect it to stay within 20% of what the pressure was on the first test (no taps running), with three taps running, repeat the tap at each property, and see if there is any variance.
This should give you an accurate idea of the flow rate, if the pressure was 2 bar initially then drops to 0.5 bar this will most likely be due to the wrong size mains pipe, and should be replaced by the water board as they cant provide you with the sufficient flow rate, if you have low pressure to start with 0.5 bar then you need to find out why.
You mentioned the water board havent been of any help, if this is still the case I would approach the Consumer Council - http://www.ccwater.org.uk/ They can advise how you can take things further.
At a guess I think you have insufficient flow rate, which was probably because they based in on a single property (not 3), and the water board will need to upsize you mains supply to 50mm.