We've just had a new screen and logic board on our 15" mbp and realise that it has 8gb of ram but would like to get more processing power. We are told it is an impossibility, but is it? Can anyone tell me if there is a way?
You need to post more info on the MBP, there are 2 different 2011 models, early and late 2011, with different specs. However, that is right, you cannot increase the processing power. However, as one poster said adding an SSD will give a definite impression of a speed increase, the difference between even the fastest HD and an SSD is a night-and-day difference in the user's perception. The whole computer feels so much more responsive. Make sure you install any SSD in the main HD drive bay, even if you add an optibay dock for a HD.
Again, unless you are doing very intensive CPU tasks, I think the 2011 Sandy Bridge MBP should be quite sufficient. 8GB of RAM should also be enough for most people/tasks, although you can install 16GB on either 2011 MBP. Check the RAM usage in Activity Monitor (/Applications/Utilities), if you get high page outs while doing your normal usage pattern, then you may benefit by adding more RAM, if you have low page outs, then you will probably not see any improvement. There is a thread about "optimizing SSD" here somewhere you should read if you do add an SSD.
In short, your computer, while getting old now, is still quite usable. If you have the cash to buy a new model (wait for the Haswell update), go for it, but it is probably not really necessary.