That's pretty good, Rachel.
Now, I'm no expert on cover letters, so I can't comment on the overall structure (though it appears to match what I've seen others mention). I will instead call out a few points where verbeage/punctuation could be tighter, or you are repeating yourself.
I am not sure you need to mention the W. Yorkshire setting, unless that is relevant to the story. That it's modern-day can be inserted as contemporary urban fantasy.
The first paragraph of story summary doesn't work for me. …
The world as we know it has been subject to a Reality Filter. This is weak in seveal ways. It suggests that it is entirely in the past, whereas it is present (for all but Kelly). Also, technically it is passive. Personally, I'm not a fan of the capitalisation of Reality Filter, unless it's a specific device that people are individually fitted with at (or near) birth by some controlling entity.
Kelly, an assistant psychologist, just broke her Reality Filter and now sees what’s Really There, including three imps, who’ve decided to befriend her. Again, the capitalisation of Really There doesn't work for me. It feels like a cheap hack at trying to circumvent not being able to describe the obfuscated reality. I would also break the imps out into a sentence of their own.
This is extremely distracting, especially when she is trying to support people who see and hear things that AREN’T really there. Or so Kelly always thought. The first of these sentences is fine, though the capitals as emphasis somewhat undermines the reality. And the second sentence throws a curve ball as it confuses that Kelly now knows as reality without her filter, and what she thought was reality before it broke.
In the second, one of those niggly choices in …being made of shadow, one which has been hiding…; the good old which/that decision. The best explanation I ahve seen is that that should be used without a comma when the following subclause is wholly pertinent to defining the subject of what precedes, and which, with a comma, should be used when the subclause could be dropped without losing significant meaning. As your previous clause is the word one, it should be …one that has been hiding…. Or drop the one and go with …being made of shadow, which has been hiding…
…for the safety of her patient should be followed by a colon rather than a semicolon. Also, in the list, I wonder if her mental health should be the last in the list, to give it more emphasis. In Kelly's shoes, as a psychologist, isn't that more important than her sensible life?
The living in Yorkshire sentence is, I believe, irrelevant; only the Yorkshire element might matter if the story couldn't be transposed to some other location without destroying it.
Where you say Reality Filter is your first novel, you are repeating yourself (mentioned in the first line). I would be inclined to keep it here and drop the initial mention. Also, saying it's a planned series apparently is better states as "has series potential."
The synopsis… I haven't got that far.