Don't know about Poolsneighbour's eyesight, but I have often been able to identify airlines overflying us (assuming they are clear of cloud!). A relevant factor, albeit slow, is that planes using Heathrow have got bigger over the years, so their tail fin logos are bigger - and they appear lower. An old Boeing 737 has a wingspan of 29 metres. An Airbus A380 has a wingspan of 80 metres. Crudely, the brain will perceive the Airbus to be almost one third of the height of the 737!

In general, jet engines have got quieter, so the A380s of today are, I think, significantly quieter than the 707's that used to use Heathrow. Incidentally, the 707 had a wingspan of about 40m so would have appeared twice as high as an A380.

In my experience how noisy the planes appear to be depends a lot on the weather - temperature, humidity, cloud and wind direction all have an effect - as, of course, does the extent to which windows are open. I'm pretty sure that if you check on http://webtrak.bksv.com/lhr you will find that they are at the normal height of around 4500'.