Advice on wireless connection between house and garage.

[#1]

Hi all - I just wanted to pick your collective grey matter if possible. I've just had the garage converted into a study, it's about 10 metres from the house. The wife works in the far side of the house, about 20 metres from me (not far enough she probably thinks). I'd hoped that my D-Link G604T ADSL modem/router would be able to fire a decent enough signal for her laptop to pick up, but not a chance.
The router is in the garage and the signal has to travel through the garage roof and two layers of foil-backed insulation to get to the house. Once at the house it has to travel through the external wall, then 3 more walls and a staircase - it can't make it. I bought an AP box (Belkin Wireless G Universal Range Extender) and this only just picks up the signal in the house after it's travelled through the external wall, but even then it can't get the signal tthrough the remaining 3 walls to ger laptop.
I bought a couple of those extended aerial extensions (38cm) to put on the D-Link router and the Belkin extender, but it doesn't seemed to have helped much.
Does anyone have any recommendations? I've though about just getting a better router (I've heard that the D-Link is a bit underpowered) and maybe that along with another Extender in the house might do the trick. I'd also thought I could mount an external box or mast on the garage so the initial signal can avoid passing through the garage roof and insulation. Another option was these boxes you plug into the mains and the signal is passed through the mains but with the garage being on a separate loop (though it goes back to the same mains fuse box via its own smaller fuse box) I wasn't sure they'd work. I've also wondered if I could just lay an ethernet cable from the garage to the house under the garden, though I've no spare on the router so I'd have to buy a hub or something. I've also read about the new 'n' specification wireless, though neither of our laptops are compatible with it so that would mean a couple of new wireless adaptors. The other option is to just get another connection in the house with plus.net, though I really want to keep us all on the same network for backup reasons (my backup is in the house so if one burns down at least I have some work in the other!).
Any help seriously and greatly appreciated - there are so many possible avenues to plunge down but without knowing which is the best/most likely to work it's hard to jump in.
On a separate note, I'm having a few issues with my connection since transferring the account from the house number to the garage number (two phone accounts). I used to sync at 1152kbps downstream but now I'm only getting 608kbps. The BT site says my IP Profile is 350kbps. The router states:
Connection Status Connected
  Upstream Rate (Kbps) 448
  Downstream Rate (Kbps) 608
  US Margin 8
  DS Margin 3
  Modulation MMODE
  LOS Errors 0
  DS Line Attenuation 63
  US Line Attenuation 63
  Path Mode Interleaved
Not sure what they mean but I assume these are important diagnostic details.
The symptoms are disconnections during the day (we had about 15 yesterday) and spikes in the download speed.
Just for your information I've now got the router plugged into the master socket's test socket and swapped the microfilter out as the phone seemed to be messing with the connection yesterday also.
We're also about 2.1 miles from the exchange and I know bad weather used to drop the DL speed on the home phone.
Again, any help from you chaps at plus.net your end to see if this new line is dodgy would be appreciated!
Think that's about it for now
kopper

[#2]

A LAN cable is by far the best solution. Hubs are "cheap as chips".
It also looks like you'd be better going back to the "house" line for broadband.

[#3]

@kopperdrake
See you have thought about ethernet over your power lines. Unless your house and garage are on different phases (most unlikely) then all the circuits will go back to the same fusebox/meter and system will work well - trust me  ;).  Have a look at http://www.devolo.co.uk/uk_EN/index.html or http://www.solwise.co.uk/net-powerline.htm or http://www.homeplugs.co.uk/. Usual caveat, no commercial interest, just a satisfied user of Solwise HomePlug AV 200s  :).
Regards,
Ray 

[#4]

And don't forget that ethernet (over power or plain cable) works both ways, so the router etc. can go either end.

[#5]

Thanks people! How does this system sound to you then?
ADSL connection and router in the house so the laptop can plug directly into the router and no doubt the wireless will work there too with both laptops if the wife roams from her study and I decide to browse on my laptop rather than watch coronation street.
Then from the router an RJ45 to one of those Solwise Homeplugs plugged in in the study. The other Homeplug plugged in in the garage with an RJ45 to a hub, from which I can run cables to the four PCs.
Sounds like an option?

[#6]

Yes, and if it doesn't work get digging for a proper RJ45 link! 
Obviously test the power link before you move things around.

[#7]

Hehe - thanks for the reminder - out of impatience I'd probably have just started lugging everything across
Thanks again for the heads up - I just realised I could probably plug my Belkin Extender into the switch/hub in the garage to get wireless in there too as an AP, as the D-Link modem/router has wireless already so the house would be covered! Win win

[#8]

Good thinking. You should always have a solid "backbone" network first, then add the frills (wireless etc.) wherever required.