You were only getting an annual letter while you were picking 1 yr fixed options, when you changed to 3 yrs then the letter would have come out at the end of three years. What I mean is there is no annual options letter in general, you were getting it annually because of what you were picking.
You didn't start on a tracker or have a fixed initially with a default to tracker when it ended.
You got the option of tracker the years it existed, the bank were just offering you what rates they had that were available to you and in some years there was a tracker as well as fixed options. The tracker ended mid 2008 or so therefore it didn't appear anymore on your options, equally if fixed rates had finished they wouldn't have appeared on the options letter either or if some other new type of mortgage rate was available such as a loan to value rate that might be offered.
Fixed rates were never guaranteed to be always available either but obviously they were during those years even if the fixed terms changed, for example there wasn't always a 10 yr fixed available.