Warning This Section Does Contain Spoilers
In this letter Heather says the word "Psych" and seems to be in the slang term meaning "Not!" or "I'm kidding!". If so then this letter would have to have been written after 1983 because that term came to be from the Eddie Murphy's stand up comedy "Delirious" in 1983. Coupled with the talk of cassette mixed tapes which were still very common at this time, this letter would be valid from 1983 up to the early 90's when people started relying on compact disc almost exclusively. In Western Europe and North America cassette tapes declined sharply after its peak in the late 1980's. Other meanings of the word are psych meaning to psych out, normally reserved for a sports setting and beginning use was dated to the 1930's. And also psyched like being excited about something and dates to being used back in the 1960's. A fourth but almost unrelated option is psych as in psychedelics which was a nickname used for drugs in the 1960's.
This would put JWJ's transfer to Darlington after 1983 but before Lloyd McGowan's transfer to Darlington in 1997 most likely.