Some people have been affected by "Advertiser's Puff". There is NO SUCH THING as a Digital Aerial. This is just something that some aerial installers say to sell more aerials. You already have a High Gain one, and that is what you need (if it's the right Group or broadband).
First: is your downlead split to feed your telly's using a "passive" (unpowered) splitter? If it is then you could be suffering from reflections within the various branches of your cables. Try just using a single unbroken lead down to one television and see what happens.
The aerial will be optimised to feed a 75 ohm load and that can only be met by having a single lead (without branches) to a single receiver.
Splitting the cable causes conditions that can not only reduce signal strength but also cause distortions which affect digital signals more than analogue ones.
Second: look up your nearest transmitter. Use the link below to the BBC site for information. Note that you may be in a 'fringe' area and have to use one of their Repeater transmitters.
Check that you have the correct aerial group for that transmitter.
Ascertain the bearing of your house to the transmitter and then you can use an ordinary compass to check that your aerial is pointing in the right direction.
If you have high buildings, trees or building works in the way then you will have to find your next best transmitter. If your neighbours' are pointing all over the place then it's possible that they have had similar experiences.
Third: don't waste money on a 'booster amplifier'. If your initial signal is crap then that won't make any difference.
Fourth: once you have got a good signal with your single television then you should use a powered splitter to feed your other receivers. Get one from B&Q or Maplin.
If that doesn't work then get in an installer and ask him to use your existing aerial (to save buying a new one). He'll align it using a field strength meter (cast about £500 so don't go buying one yourself). He'll charge about £100 but at least you'll have a good working system with a guarantee.