OK, this should be really easy, and I've tried keyword everything....but apparently my cross reference/keyword-fu is sadly lacking. (As is my PC, because I know I had this saved, but the HD borked itself)

Post Chosen, post NFA. Buffy buys the old Pratt house & starts fixing it up. She finds a vampire lurking in her shed, he's badly burned and non aggressive, so she takes him in & fixes him up. He has amnesia, so she let's him live in her basement & calls him 'Bob' because he can't remember his name.
Naturally it's Spike, who is eventually healed enough to be recognized (actually i think he called Dawn 'niblet' by accident and got found out)& him and Buffy go avert an apocalypse.

Any nice folk willing to point me in the right direction?

Hi, could it be Home Is Where They Have to Take You In. by slaymesoftly?

Summary: It's several years after NFA and Buffy has found a nice house in London. Bit of a fixer-upper, but it has some things to recommend it. Spike and Angel lost their battle with the Senior Partners and have been gone long enough for Buffy to have moved on with her life.  Then she finds something living in her garden shed... and demons come to town... and it's a whole 'take care of the creature in the shed and fight big scary demons from another dimension' thing.  And then it gets complicated....

Hope this is it

Hannora
You are awesome, that's the one!