Okay we have a c.Sub_End_Date__c field on the contract page (when a contract is coming to an end).  We have an existing trigger set up to update the Account page field (a.Worry_Date__c), copy of trigger below:

trigger updateAccount on contract(after insert, after update) 
{ 
List<Contract> CL=trigger.new; 
List<Id> aid=new List<Id>(); 
for(Contract c: CL) 
{ 
aId.add(c.AccountId); 
} 

list<Account> AL=new List<Account>(); 

for(Account a: [select Id from Account where Id in :aId]) 
{ 
List<Contract> ICL=[select Status, Sub_End_Date__c from Contract where AccountId=: a.Id]; 
String max='1900-01-01';
for(Contract c: ICL) 
{ 
if(c.Status=='Activated' && c.Sub_End_Date__c !=null) 
if(Date.valueOf(max)<c.Sub_End_Date__c) 
max=String.valueOf(c.Sub_End_Date__c); 
}
if(max!='1900-01-01')
a.Worry_Date__c=Date.valueOf(max); 
AL.add(a); 
} 

if(AL.size()>0) 
update AL; 
}

We now have new products we wish to monitior on the Account page we have another field named a.ESP_Worry_Date__c and from the Contract page when the c.Sub_End_Date__c field is updated and the prouducts are either 'ESP-E-R','ESP-B-R', 'ESP-B-I', 'ESP-P-R', 'ESP-P-I', 'ESP-E-I' then the a.ESP_Worry_Date__c has to be completed on the Account page.  However if the products are 'SUBS-I', 'IMP', 'SUBS-R' then the a.Worry_Date__c has to be completed on the Account page.

Hope this makes sense.

Thanks
Susan