I thought that too. I just assumed he was translating from AE or something, as they often use words and phrases in a manner I find strange.You can't really say 'fairly recent months'. Do you mean over a period of months which wasn't that long ago or do you mean the last few months?
In the first case ' Fairly recently I'd written a lot of letters during a period of several months'
In the second ' I've written lots of letters in recent months'
Recently I have written a fair amount of letters....
I have written a lot of letters fairly recently.