poor wifi

[#1]

just wondering if its possible to use a different router than the supplied fibre one.wifi performance is very poor upstairs so was thinking about maybe buying a dual band one

[#2]

It maybe worth saying which router that you have, as PlusNet have changed recently.  If it's the Sagemcom then it will not be hard to improve upon!
Dual band in itself may not provide improvements if it's a distance from to router issue the 5GHz band has a shorter range than 2.4GHz.

[#3]

You can use a different router.
However I would give the advice shown here a try before buying one: http://www.plus.net/support/broadband/wireless_broadband/wireless_signal.shtml

[#4]

yes it is the sagecom ive got anyone recommend something decent

[#5]

Here is a list of combined Fibre Modem/Routers The tread also contains advise and reviews.
You could also stick with a separate modem, which opens up you choice and reduces the cost.
I recommend going for a dualband wireless router that supports `ac` to keep yourself future proof.