If I'm understanding you correctly, what you want is something like this:

blocks/myblock/lib.php:

function block_myblock_somefunction() {
...
}

blocks/myblock/classes/task/mytask.php:

namespace block_myblock\task;
class mytask extends \core\task\schedlued_task {
   public function execute() {
        global $CFG;
        require_once($CFG->dirroot.'/blocks/myblock/lib.php');
        block_myblock_somefunction();
   }
...
}

But, you really want to avoid doing this, if at all possible.

These days, you are much better off sticking the function you want in some sort of helper class inside block/myblock/classes - then you don't need to remember to use require_once() to load the code, before you use it (you certainly don't want to be using locallib.php these days, it has none of the advantages of automatic class loading).

e.g.

blocks/myblock/classes/helper.php:

namespace block_myblock;
class helper {
   public static myfunction() {
    ...
    }
}

blocks/myblock/classes/task/mytask.php:

namespace block_myblock\task;
class mytask extends \core\task\schedlued_task {
   public function execute() {
        \block_myblock\helper::myfunction();
   }
...
}

(note you may need to purge caches or bump your plugin version number for Moodle to spot this existence of your new class)

Hi Dave,

Davo's response is a much better solution though it requires more thought and coding??? - my suggestion was the 'quick' solution - not always the best way of doing things ...

Ben