Weak wireless signal - Thomson router

[#1]

I replaced a Netgear router which performed badly on ADSL2+ with Plusnet's Thomson wireless router.
I used to be able to enforce a fixed rate 54M wireless (with WEP "encryption").
Now I have to accept a much slower (but variable) rate (but with WPA-PSK encryption).
Is it possible to exchange the little aerial on the Thomson router with a bigger higher-gain one?
It doesn't appear to be detachable, contrary to many routers.

[#2]

All aerials are the same give or take as the length is governed by the frequency. The only way to improve it is to get a higher gain one which normally means a Yagi but this is directional so may not help unless you only want it in one direction. The power of all routers is the same the thing that make the difference is the location  moving it a foot can have a big impact as does reducing any interference from TV, Microwave etc

[#3]

I've already been through all that - including changing channel.
On my previous router, I did replace the little stick with a longer stick - this kept the signal into a much tighter horizontal direction.
Result = moderate effective gain.
But I don't think I can with this Plusnet router as the aerial seems to be glued on.
I think a Yagi sounds a bit extreme 
I've got an elaborate version on a pole for my digital tv 

[#4]

There's only one thing for it then axis, get a pop can and a knife...
http://www.metacafe.com/watch/943053/increase_your_wireless_signal_strength_with_a_soda_can/

[#5]

http://www.turnpoint.net/wireless/cantennahowto.html
People have achieved a range of over a mile using one of those.
Edit: It's amazing how many hits you get if you Google baked beans yagi

[#6]

I can't do a Cantenna - but adiewoo's sounds excellent - even a small increase in signal would be good.
It also has the advantage (for me) of blocking broadcasts to the road outside.
I shall definitely try it out, once I have put a shelf up (to increase the height of the WP).
Thanks.

[#7]

The Thomson has a fixed antenna - as you presume - so you can't add an high gain antenna.
With routers that do have a antenna socket - yes you can add a higher gain antenna, and there are many types - as suggested yagis, but ther are other options such as colinears - like here:
http://www.panorama.co.uk/uk/products/wlan2.4_terminal.html
the 4dBi WLAN Terminal Antennas