Graham, We can help you figure out what possible fixes, upgrades and tweaking to do to help get your machine back up to speed and actually running better than new.
What we need to do first is find out exactly which machine you have. You can do that by going under the Apple to "about this Mac", There are known issues on different models. There are hard drive size and type upgrades along with system upgrades.
Your machine is like your body, if you stick a bunch of junk in it it gets fat. Things get put into your start up items, your hard drive gets full, you can pick up little viruses. The fan vents, like arteries can get clogged and your CPU and GPU start overheating because it takes more effort to run with clogged arteries.
Now you have gotten with the best Mac doctors in the world and this is what we do and know very well. Give us a history just like you would to your doctors and we can tell you the things you can do and possible upgrades to do to get you healthy again and give your machine a much longer and productive life.
So what, machine do you have, how big and how full is your hard drive, how much RAM do you have, how clogged are your arteries, have you had your flu shots and vaccinations?