Here's a good'n.
If you don't know it already, work out your grid reference using:
http://www.streetmap.co.uk/
Submit your postcode and then, at the bottom of the map, you'll see a link called 'Click here to convert/measure coordinates'. Follow that and you'll see your Landranger grid reference on the row marked 'LR'. Copy that and then go to:
http://www.kswindells.34sp.com/freev.../transmitters?
Pop in the grid reference, select your transmitter and away you go. It draws a profile of the ground between you and the transmitter. Ideally, there should be no obstacles above the red curve.