Is there a way to 'lock' the phone without having to open/close the slider?
Locking the phone?
Yes, if you press the left softkey then right softkey (the top left and top right buttons) when looking at the phone application(where you dial)/idle screen this should lock your keypad.
Should be able to do it from the power menu too. Press the power button, you'll have 2 locking options, 'Lock Phone' and 'Lock Keypad', you want 'Lock Keypad'.
Thanks all, v much appreciated!