In think your suggestion of a water tank is a reasonable option, you need to calculate your peak flow for all three properties (eg if someone had a shower in all 3 properties), this will determine the size of the water vessel you will need (if you want one water tank supplying all properties).
I would be inclined to have a gravity fed system per house hold (a tank in the loft), you can then have two sources of input (private and mains), with the private being the primary inlet, and the mains maintaining the tank at 33% for example.
Depending on your combi boiler you will probably need a pump, any power shower pump will be sufficient as your just looking to achieve 2 bar, this then means you will have sufficient pressure on all taps and not adversely affected if the two cottages both turn on the shower, as each house hold has an independent supply.
Please let me know if this sounds like it meets your requirements, and if you need anything clarified.
The challenge is you need consistent pressure and secondly adequate water supply feeding all taps (hot and cold).
I would look to use the gravity system in your barn, and get someone out to look at what you would need to keep the combi boiler happy, it may be you can get away without using a pump all together.
With regard to the other two cottages, I really would look at how you can redirect the feed, it shouldnt be a lot of effort to run some MDPE 6M vertical into the loft, surely you have pipes going up for the radiators from your combi, or soil pipes going through the roof?
The key thing I would stress is avoid having a water tank below ground as you run into issues with contamination and all sorts.
Maybe for the cottages you just reside to use 100% mains water, and if they cant supply you with adequate pressure put an inline pump at source (under the kitchen sink).
Please let me know if I can be of any further assistance.
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.