Given that there are so many pixels remaining, it may be worth offering more for the £5. From a marketing perspective, as a company you want to do most of the work for the customer to create the minimum amount of hassle for them.
For their £5, they get a barely visible image on a webpage. Offering them bigger images for their fiver will allow people to see the logo without having to hover over it. Obviously if this was to be implemented, it would have to be kept on the quiet from those who have already purchased pixels as they may wish to get more for their money.
Also, companies always like to get a handle on what sort of figures they are likely to be hitting so (if it hasn't already been set-up) it may be worth setting up a Google Analytics (
http://www.google.com/analytics/) account to give precise figures to those interested: it makes it seem a lot more professional as well. If the number of hits on the webpage isn't quite as good as expected, the figures can always be manipulated to make them look more attractive to those who will invest.
Given that this has been running for a fair amount of time already, it would seem that those wishing to donate for the cause will already have done so. The main angle should now be to try and entice potential investors for their benefit as well as RDFC's.
Having so many unsold pixels is no good for anyone so get ride of as many as you can for whatever price people are willing to pay. The club is in such a dire financial situation that we should just take what we can get at this moment in time.
Everywhere we go....