firstly the links you posted just go to the home page for maplin - but this is no big problem.
Thermal compounds do vary, but there are a couple of brands that most people trust the most.
Arctic Silver 5 - performs very well, but the compound does have a period of time before it reaches is best performance - around 100 hours of use.
MX2 / MX3 are just as good and do not require a wearing in time like the AS5... also they are non conductive so if you do accidentally have the compound come into contact with electrical contacts it wont produce a short.
Yes, you have to remove ALL the old thermal compound before you put on the new, on both the CPU and the Heatsink. Most people use Isopropyl Alcohol an a lint-free cloth. then follow the installation instructions supplied with the new compound.
I've just sorted out my system and thought I should just post an update ^_^ I now have a nice cool CPU
Before I was reaching 70-80 Celsius even in the BIOS and an idle temperature of 60-70 and assumed it was cause I didn't apply any thermal cooling so I went out and bought some, removed the CPU cooler and noticed the stock thermal compound I removed it and applied the new one but it only knocked off about 3-5 degrees, So I bought a new a CPU cooler and now I'm at an idle temp of 35-38 Celsius ( ) and haven't gone above 45C in an MMO highest quality settings with things running in the background as well!
So far I have yet to see the CPU's temp reach anything above 45C so I'm happy