Serving a document means delivering it in a way that you can later prove it was delivered. Without proof of service, your strongest letter is just paper you wrote and never sent. With proof of service, every day of silence from the other side starts to count.
The standard method in Britain is Royal Mail Signed For or Special Delivery. You keep the receipt with the tracking number. You also keep a stamped copy of the document and the envelope address. That is your proof.
For documents you want to weight more heavily, hand delivery with a witness works. The witness signs a short statement: on this date, at this time, I saw [name] hand-deliver the attached document to the office at [address] and obtained a receipt or noted the name of the person who accepted it.
The clock
Once a document is served, the clock starts. Most notices give the other side twenty-one or twenty-eight days to respond or rebut. If they do not, you move to the next stage in the sequence: a Notice of Default, a Notice of Estoppel, or whatever the original document said would follow. The dates are not arbitrary. They are part of the lawful structure of the exchange.
Keep a single notebook or spreadsheet for every matter. Date sent. Method of service. Tracking number. Date received (from the tracking record). Deadline for response. Date of any reply. Date of your next letter. Without that record, sequences fall apart and the other side runs the timing instead of you.
Hold the line
The other side will sometimes claim they never received your letter, even when you have a tracking record showing it was signed for in their building. Do not get drawn into an argument about that. Send the next letter in the sequence with the original tracking record attached as proof of service, and continue. Their internal mail handling is their problem, not yours.