How long's a piece of string?
The parents' house is a nightmare; either Biggin Hill airport has installed some bastard-strong radar on the 2.5Ghz band, or one of the neighbours has some shite non-DECT approved cordless handsets which are interfering with the signal; you simply can't run encryption (very bad), and range is less than 8 metres with the best kit.
Down here in Pompey, most folks don't know what cordless phones and wireless are, and I can get up to 40m range on similar kit.
So the short answer is "no-one knows" - experiment with placing the wireless access point in different locations. Stairwells are a good starting point, and make sure it's at least 3 feet off the floor if possible, and away from metal and other electronic doohdads, e.g. cordless phones, microwaves, etc.
/al