HI Geoff,
Getting differing make wireless gear to play ball with each other is a nightmare..
Your BT kit is one of the weakest wireless sigals I've come across. and a bugger to get to talk to other kit...standards we're talking BT here.
Best option to buy a Buffallo airstation whr-hp-g54 about 50 quid (best I've found real world strength wise) disable wireless on the nasty BT box plug a RJ45 patch cable between it and the buffalo and away you go; should have no problems in a three story house use these in businesses all the time they cheap and reiable.
If you still need to repeat Buffalo stuff is easy to set up, but I'd be supprised if you will need to.
I know they aways say put the adsl router on master socket but that is nearly always downstairs in the hall (shite place for wireless) you can nearly always move it to an extention higher up in the property, my own one is in the loft give really good coverage over three storeys and 150 foot of garden and garage.
Other things that affect the performance of wireless,
Wireless Phones keep as far any from base stations as you can.
Differing makes of wireless clients
Differing speed wireless clients play havok with overal speed,
If your not working on state secreats, don't bother with any more than 64bit encription, lots of installs just use MAC codes to prevent others getting on the network.
R's
G