For starters, I'd recommend you set a default value of 0 for the numerical fields on Lead. Here is a rather simplistic approach to thigns - i've assumed you want this only on Insert. Also, this would not cater to people deleting tasks after they've been added to the aggregate fields on Leads.

trigger TaskAfter on Task ( before insert) {

Set<Id> leadsToUpdate = new Set<Id>{};

String leadPrefix = Lead.SObjectType.getDescribe().getKeyPrefix();

for(Task t : trigger.new)

if(t.WhoId != null && ((String)t.WhoId).startsWith(leadPrefix)  //if not null and whoId belongs to a Lead

leadsToUpdate.add(t.WhoId);

Map<Id, Lead> leadMap = new Map<Id, Lead>([Select Id, Phone_Call_Attempts__c, Successful_Phone_Calls__c,  Future_Dated_Task_Scheduled__c, Overdue_Task_Flag__c from Lead where Id IN :leadsToUpdate ]);

for(Task t : trigger.new)

{

Lead currLead = leadMap.get(t.WhoId);

if (currLead != null)

{

if(t.Status == 'Completed')

{

currLead.Phone_Call_Attempts__c += 1;

if(t.Outcome__c == 'Call Successful - Spoke with contact')

currLead.Successful_Phone_Calls__c += 1;

}

else if(t.ActivityDate > Date.Today() && t.Status == 'Not Started')

currLead.Future_Dated_Task_Scheduled__c = true;

else if (t.ActivityDate <= Date.Today() && t.Status == 'Not Started')

currLead.Overdue_Task_Flag__c = true;

}

}

if (leadMap != null && !leadMap.values().isEmpty())

Database.update(leadMap.values());

}

public with sharing class LeadTaskController 
{
public static void UpdateLeadTaskComplete(List<Task> lstTask)
{
	
	List<Lead> lstToUpdate=new List<Lead>();	
	Set<Id> setLeads=new Set<Id>();	
	for(Task objTask :lstTask)
	{
		setLeads.add(objTask.WhoId);
	}	
	System.debug('---------task--------'+lstTask);
	System.debug('----------set-------'+setLeads);
	try
	{
		List<Lead> lstLeads=[Select l.Phone_Call_Attempts__c, l.Successful_Phone_Calls_c__c, l.Id, l.Future_Dated_Task_Scheduled_c__c, l.Four_Hour_Follow_Up_Time__c, l.FirstName From Lead l where l.Id in : setLeads];
		
		System.debug('-----------lstLead'+lstLeads);
		for(Lead objLead: lstLeads)
		{
			for(Task objTask : lstTask)
			{
				if(objLead.Id == objTask.WhoId	)
				{
					//Condition 1
					System.debug('----------------Task--------'+objLead +'-----------------Lead'+objTask);
					if(objTask.Status=='Completed')
					{
						if(objLead.Phone_Call_Attempts__c !=null)
						{
							objLead.Phone_Call_Attempts__c=objLead.Phone_Call_Attempts__c +1;
						}else
						{
							objLead.Phone_Call_Attempts__c=1;
						}
						//Condition 2
						
					}
					//Condition 2
					lstToUpdate.add(objLead);
					break;
				}			
			}
		}
System.debug('---------------'+lstToUpdate);
if(lstToUpdate.size() > 0)
update lstToUpdate;
	}catch(Exception ex)
	{
	lstTask[0].addError('Exception--'+ex.getMessage());
	System.debug('----------------'+ex);	
	}
}
}

Trigger Code

trigger UpdateLead on Task (before insert) 
{
	LeadTaskController.UpdateLeadTaskComplete(Trigger.New);
}

Hi - thanks so much for reply!

am trying this in sandbox in task triggers but am gettnig the following error:

Error: Compile Error: No such column 'WhoId' on entity 'Lead'. If you are attempting to use a custom field, be sure to append the '__c' after the custom field name. Please reference your WSDL or the describe call for the appropriate names. at line 10 column 43

Oops, the perils of type in textpad

WhoId should not be in this SOQL Query

Map<Id, Lead> leadMap = new Map<Id, Lead>([Select Id, Phone_Call_Attempts__c, Successful_Phone_Calls__c,  Future_Dated_Task_Scheduled__c, Overdue_Task_Flag__c from Lead where Id IN :leadsToUpdate ]);

This works perfectly thank you!

The only thing is when the 'Future Dated Task' is closed off as completed - it does not 'untick' the Future Dated Task Scheduled box.

Is their an easy way to do this?