What size are they, i assume you can just replace them like for like or is their anything to watch out for i.e depth?

erm, which car is it?

usually you need adapters to make aftermarket speakers fit. autoleads make many adapters.

you can get aftermarket speakers that fit without adaptors but you need to get correct speaker size and mounting depth.

tis a mk2 punto so speakers required are 17 cm componants. Mounting depth is crap in them i think its about 45mm or something so you will need either autoleads speaker pods to give you better mounting depth or make your own spacer rings to give you more depth