My original question was can the signal strength be improved by moving the aerial from line of site? Looks like it's not worth the bother trying.
And the answer is - possibly. Depends on too many factors and needs to be trial and error. Usually beneficial only when not in line of sight and signal is low. You may be being overpowered with VHF analogue signal. If the antenna and cable is old they should be replaced. You may have an open circuit feeder? Have you tried an indoor antenna or even just plugging a short length of wire into the TV's antenna socket? I'm about 7km line of sight here in Auckland and can get a useable signal on a short piece of wire on the end of a short co-ax lead. Moving my antenna thru 360 degrees doesn't seem to make any difference to the signal.
As the cable and antenna are old and some connecters look corroded that is the likely cause of the poor signal not the alignment. The VHF overpowering the digital signal will go when it's switched off in future. Don't think I'll bother with an indoor antenna as the house is soon to be sold, I have learned a lot from all the helpful responses given here.