Airport Express - running out of ideas why it's suddenly so slow

I'm hoping someone knows what's going on here - there must be something obvious that I'm missing.

The facts are:
- When I plug directly into my Billion ADSL modem via Ethernet, if I run a broadband speed test, it's up around 7000 kb/s, which is our ADSL speed.
- When I plug the Airport Express into the Billion modem and run a broadband speed test, it's stuck on about 230 kb/s.
- Strangely, however, I can run two, three or four speed tests via Airport and all run at the same speed.

So it seems that there's no issue with speed to the Airport, it's almost as if it's limiting the size of each individual connection?

Here are the (unsuccessful) steps I've taken to diagnose it:
- Changing the modem to bridge instead of routing - no change
- Toggling interference robustness - no change
- Turning off our cordless phone and changing wireless channel - no change
- Resetting all modem and Airport settings - no change

The speed slowness is being experienced across all our laptops - both Mac and PC - and it doesn't matter if only one person is online or all of us.

Anything else I should be trying? Is there something obvious I've missed in trying to diagnose it?