If I am calling an overseas mobile phone do I need to add the country code for where it is ?

yeah, you replace the leading 0 with the country code number.

But as far as i'm aware the country code number depends on where you got the SIM, I think UK numbers are always +44 no matter where you are in the world.

If you're calling a number that is usually based in the UK (eg. a friend who has taken their phone on holiday), then you just dial as normal.

If you're calling a number that's actually registered abroad (eg. a friend who LIVES in a different country) then yes, you do have to put the country code.

Many thanks