I was given a DVB-T signal meter for my recent birthday, so I decided to try some tests with this. The meter has 4 signal level LEDs, a power LED, switch and loop-through for a receiver. On the setup I used in this thread with the attic aerial and amplifier, a single LED lights up:
It turns out that the meter will also light a single LED if I connect a short cable between an amplifier and the meter, no matter which way the aerial is pointed, so if anyone plans using a DVB-T meter, the aerial should be connected straight to the meter without a signal amplifier in-between.
I tried out the meter while heading with someone to Sligo, so for my own curiosity I took my TV-top aerial and checked the signal throughout most of the journey to see what areas pick up the best reception (results here). When I connected the attic aerial directly to the meter, no LEDs lit apart from the power LED, so it seems like that as long as a single LED lights, there is decent DVB-T reception, especially since there's a large difference between the gain of a TV-top aerial with no amplifier and a full size aerial with amplifier.