I've a little house in a valley and my TDT signal has always been pretty poor,but of late it's gotten worse.Channels that I can't receive in the morning I can get clearly by late evening.The house next door is maybe 15 ft higher and gets full signal all day.As the transmitter doesn't move and the house stays fairly still,any ideas as to why my signal fluctuates?

you have answered the question yourself....

but here is the tecky bit......
......this is caused by a phenomenon called "RF Propagation".
untechnically - think of a bubble or baloon, where the inside is a fixed volume and when you press a portion it reduces at that point but the air has to go somewhere so there is a resultant expansion in other areas.

when the sun shines on the ionisphere it contracts, forcing radio signals down reducing the coverage, when it cools radio signals rise increasing the coverage..
OK, so the frequencies involved for terrestial television are generally "line of sight". But you are seeing the overall effect of the sun on the signals.
you also have the problem of that downward pressure meaning that a house, rock, tree, mountain, is blocking the signal.

may I suggest that you get an extension to the antenna pole and raise the aerial higher (or see if you can re-install it on a different location that gives better line of sight). Also, you may wish to check the signal strength(s) on the TDT info section and "tweak" the antenna to the highest you can get.
(remember that if you are following the house next door they may be a little bit out but it doesn't matter as they have a better line of sight so no drastic loss in signal strength)

Changing from analogue to digital although increases drastically the quality and more services from the signal, means you have to be more accurate in the direction of the antenna, and modern flat screens need a higher signal level than the old analogue's.

something that may also help is to get an antenna that has a higher dB output from the signal. This doesn't just mean fitting an amplifier (I hate them) as they will boost crap as well as good. A higher dB output doesn't always mean more expensive, just a different method/materials of construction

Tecky bit over, I hope you get it sorted, nothing worse than a pixelating
kr

Hey that was good stuff - we suffer (sometimes) as well and we live in a big town with the antenna on top of a block.

single antenna to you only or a single antenna serving X flats ??

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Yes for the communal TDT system - it goes via booster box, which blew up in a storm last year - no TV for a week.