There are a couple that I would recommend.
Kathy Brown wrote a wonderful continuation of this for the 5th Season project (which are all fantastic reading, by the way) and it's called 'Through the Looking Glass.'
I'd also recommend Margaret Brignell's fantastic 'Only You' 5-part series.
Jeff Brogden wrote 'Then Came You' which is also wonderful.
And I wrote 'Trading Places' which is another exploration of this episode.
There are a LOT more, but this will do as a start. I'm sure that others will make more recommendations as well.
Irene