I'd just like to say a big thanks to Holly, Krista, John Dillon, Quill, mfreivald, atdeluca, bookwatcher, and adam73, (and basically everyone on these forums), for their great advice and help in whipping my query into shape.
Good news is - the query worked! I am now represented by John Rudolph of Dystel & Goderich Literary Management and THE HOUSE OF MIRRORS is now out on submission. Thanks Folks! Apologies for the delay in coming back and giving you the update, but life has been a touch hectic lately.

***UPDATE ENDS***
Hi all.
I posted a query for this story at the end of last year, but now my revisions are coming to an end, I've given it another shot. I'd love to hear what anyone thinks and any improvements I can make to it.
This my query so far:
Hello Mr. agent Man/Ms agent Lady
Twelve year-old Sarah discovers that magic isn’t a load of guff when her little sister, Jane, disappears before her eyes. Hot on her heels, Sarah soon finds herself sneaking into an invisible mansion, scuffling with a thuggish teddy bear, and entering another world that exists through a magic mirror.
If she can’t find Jane and escape the nightmare kingdom, she’ll be forever trapped on the other side of the glass and chased by the terrifying Hunters.
Standing in her way is the Old God, an ancient creature who made the Hunters and rules this land. He wants children’s souls to power his world and bodies he can shape into books for a little light reading. But Sarah has other plans, and sets out to beat the Old God, save her sister from living the rest of her life as a paperback, and discover a way home.
Sarah’s journey will take her through a dangerous magical land, where she will meet both monsters and friends, though sometimes she'll find it difficult to tell which is which.
THE HOUSE OF MIRRORS (52,000 words, for 9 year-olds and up), is a contemporary fantasy adventure about courage, friends, and the importance of not believing everything you see.