So i went to my local garage today, and i had the coolant replaced with new stuff. They emptied all my old stuff out and replaced it with blue antifreeze (is this ok??) 3.5 litres of antifreeze and 3 ish litres of water. They charged me £15 all in, which i think is really really good, i didnt think it was this cheap, so i highly recommend it to anyone, especially if you dont know when it was last changed, or what the mixture is at, might be 50-50, could even be as weak as 30-70 coolant/water, especially if youve topped up with water a few times (or a previous owner has). Thanks for the replies Tabetha... im happy and i feel it was money well spent for peace of mind. Edit: The mechanic said my old coolant came out dirtyish, so that was surprising, it looked quite clean to me when it was in the expansion bottle, it must have been some deposits deep down in the system that i never saw, so having them all cleaned out the way can only be a good thing i would imagine.