Wireless getting weaker...Thomson v7

[#1]

I've been with PlusNet since October 2010, and right from the start it was perfect. I have to have my Thomson 585 v7 router downstairs in the hall to use the master socket, and I wanted to use wireless for my old PC and my laptop upstairs. I have health problems that means I have to lie down a lot and so wireless was the best thing for me to use my laptop with.
From my very first day the signal was great, "Excellent" in fact. However since the new year my speeds were really down. I haven't altered the position of anything, or have any new gadgets in the house, but the speed using wireless upstairs is often too slow to use and seems to fluctuate greatly from second to second now.
To begin with I thought it might be my router , or the broadband itself, or the phone line or something and I did the usual checks and turning the router power off and restarting the router, and I thought about testing the speeds using the ethernet cable downstairs. It was then I discovered that my speed wirelessly was very fast when I was only a couple of feet from the router, when a couple of minutes before upstairs it was severely slow(the ethernet cable showed I had normal expected speeds). So it appears to be the wireless signal getting weaker or something, all I know is that it's much slower and more hit and miss than when I was using it in the first few months of use??? .  I often get disconnected now and have to run the "Troubleshooting" option to try and reconnect.
I'm not computer techy at all, and I don't understand half of what is posted here,  but I think I have the up to date Firmware for the v7, using 8.2.6.5. My laptop shows my signal to be "Excellent" still, even when it's slower than a snail. Is my router going faulty on its wireless capability? What can I do to stabilise the signal? I've heard about signal boosters and repeaters, but it's all fuzzy and confusing to me. Any ideas or suggestions would be appreciated? (Providing I can understand the jargon!!)

[#2]

Welcome to the forum VictoriaIceni
For a start download and run inSSIDer which will show you your and other wireless networks in your area.
If there are other strong signals on the same channel as yours you can find and select a different channel.

[#3]

Many thanks for your fast reply. 
One strong signal nearby has the same channel as me. I'll fiddle around with new channels for a day or two and see if that improves things.
Are there certain channel numbers that will work better for my particular router? Or is any channel number OK?

[#4]

No particular channel works best with a router but channels 1, 6 and 11 are best to try. If you are using 1 and channels 6 or 11 are relatively clear chose the clearest one.
Another thought, is your wireless secure preferably using WPA2. The reason I ask is someone could be piggy-backing on your wireless and sapping bandwidth.

[#5]

Yes I'm on WPA2.
I tried channel 5, and that seems a bit better. But I'll definitely try others to see what is best and if that solves my problems.
On a note of minor security, I've never been able to change the SSID though....every time I've tried it won't stick, causes connection confusions and I have to go back to the original SSID and leave it as that.
Many thanks for all your help so far. 

[#6]

When you attempted to change the Wireless SSID did you click the Apply button at the bottom of the page? The new setting won't be saved (stick) if that isn't done.

[#7]

Yes I used the "Apply" button, but the SSID I choose never shows in the available connections list and stays as "Plusnetwireless" the next time I go to it.
I suppose if someone was trying to use my connection without my knowledge that changing the wireless key password might help to rule that potential issue out on the checklist of possible problems?