It certainly sounds like an aerial problem. If you are in a weak reception area the 'professional' installer should have known that and suggested a mast head amplifier. He/she should also have chosen the correct aerial for your geography. That is to say some areas require a special 'wide band' aerial for Freeview.
To see how good a Freeview signal you are likely to get try www.freeview.co.uk/availability
I suggest that you borrow a Freeview Set Top box. (Or even buy one because you will be able to use it to 'convert' an old TV and they are so cheap these days.)
Use the set top box to see if you have the same (or similar) problem. If so it is the aerial, if not it is the TV and so you will need to take it back to Aldi for a refund.
T
December 2008