Maybe the wiring in the house is not as you think.

When using the short grey cable did it look like the grey lead in this picture http://www.thinkbroadband.com/images/reviews/billion...

and was it plugged into the same socket as the extension lead had been?

I believe so, and yes I added the grey lead into where I previously had the long cable plugged into.

Daft Question but do you have a Sky Box anywhere around the house and have the engineers rigged these up with telephone sockets.

If they have then pull the telephone leads out and test your internet connection again,personally i never use them and never have in the past 15 years.

I say this because at one point we had a new Sky HD box delivered and i went into the kitchen to make the Sky Bloke a drink,only took him a few minutes to replace the other but little did i know that he connected it to a telephone socket and for the next week i had massive problems with my download speed which took me ages to figure out why,only by chance about 5 days later and many calls to my isp did i spot the cable behind the tv.

When removed everything was spot on and my connection was back to its best.

Might have nothing to do with your situation but i thought i would mention it.

Just to add to this.

Some people when they use an extention cable have a filter plugged in at the socket, and then another filter at the end (as they use a phone near the main socket and the broadband router elsewhere in the house).

Can you tell us if you have 2 filters connected to that extention cable?

In reply to a post by Kaytfoh:

Can you tell us if you have 2 filters connected to that extention cable?

There isn't a problem 'double filtering' if that's what you are saying?