My set-up:
MacBook and iMac both running Snow Leopard.
Sky(Netgear) wireless router/modem->(ethernet cable)>airport extreme->hard-drives used as Time Machine back-ups.
iMac closest to airport extreme.
Airport express midway between airport extreme and downstairs room.
MacBook roaming from downstairs to upstairs.

My Problem:
I bought airport express to extend the range of my airport extreme wireless network into a room which had poor signal.
It's a room downstairs furthest from my airport extreme and it's where I use my MacBook.
My iMac is in an office room upstairs closer to my airport extreme.

I used the latest version of airport utility to set up airport extreme checking the box to create a wireless network and the box to allow it to be extended.
I did the same for the express but checked the box to extend a wireless network and checked the box to allow wireless clients.
For both devices I chose the same wireless network (that from the extreme).

Although I can access the internet from my MacBook better now, I can no longer find or access either my iMac or the hard drive attached by ethernet to the airport extreme whilst in the downstairs room.
Which means I can't transfer files and can't back-up using Time Machine.
If I move back up to the upstairs room then the MacBook finds them again.

Any suggestions?
Could it be anything to do with DHCP or IP addresses?