Just noticed this cooling reservoir and was wondering if it's just an overflow for the radiator or whether it should be full of coolant because at the mo it's not.

thats the expansion tank - if u look on the back there is a sight glass - if its empty then top it up - toyota long life coolant.

Should be 3/4 full, if you fill it to the top an extra coolant will overflow out of the system. If you that you are loosing coolant, then you may have a leaking waterpump and/or leaking waterpump by pass pipe.

It's defo not leaking from anywhere and the rads always full so I guess I just need to top this expansion tank up, cheers guys

Done, thanks again.