First, I really question that Letter_Ref should be a field in your table. Whats its purpose? Even if it is necessary, I don't think the 'B' portion is. That designates that it is a Block letter correct? If so, you don't need to store that because you know its a Block letter by Letter_Type. Does the numeric portion of that correspond to other data you are already storing as well?
Second, explain to me what you think 'Letters' contain. Is it a schedule for sending out letters, or is it a tracker of letters sent? Do you hope to do both of those things or just one of those things (plan and track)? As it is, you're data is just allowing one of those things and your explanations have confused me to what that is. Is this database for scheduling or tracking letters?
The Ref B0001 gets printed on the labels for the letter for our internal billing (our department pick up the codes to allocate the billing of postage)
I apologies if I didn't explain correctly. I'll explain from start to finish in clear terms the purpose of the DB. (some may be irrelevant but maybe not)
The outlying objective of the database is to contain a record of all of the landlords we're canvassing and their respective properties. We enter the landlord onto the DB with their property details. (can be multiple properties for same LL)
The user will run a report daily on the database which will tell us which letters need to be sent out and to which landlords.
The database produces a report/list of LL's (Landlords) who are "due" LetterX
(Xbeing a number in a series of letters) The database will then allow a word document to merge the contents stored into the word template creating a letter (or label in most cases) the report will be for each letter individually, ie Letter1 Report, Letter2 Report. This is because different users are responsible for different letters. This also means the letters printed 1 at a time, so the template for Letter1 is merged, printed, updated then letter2 is done separately. (not in one go)
Once the letter merge is sent it needs to document that the letter has been sent for auditing purposes, documenting the letter has been sent will also be used to confirm the next letter is due (based on when the previous is sent).
An example letter schedule is as follows
Letter1 is due day 0
Letter2 is due 14 days after letter1
Letter3 is due 14 days after letter2
Letter 4 is due 21 days after letter3
The cycle continues for either 30 letters (Canvas) or 15 letters (Block)
Both Canvas and block use different frequencies for the period between letter sends.
Does this clear up the purpose of the DB? apologies if not clear originally?