Patch had seen the poster for the eve in our local Waterstones and was keen to go, even though at the time we hadn't read any of the Harry Potter books. So we started the first book and, armed with the first few chapters worth of knowledge, went along to Waterstones one dark wintry eve.
Patch happily decided on the school uniform-wearing Harry, so the trousers and polo shirt stayed on from school together with a school uniform jumper (though not P's school uniform so we purchased one), some glasses from the dressing up bag (put to use here as Where's Wally), a wizard's cape (see here) which is not at all true to the Harry Potter look but still a wizard's cape and P was happy so I wasn't suggesting otherwise, and a twig -sorry wand - from the garden. I really should have ironed that cape.
The school jumper needed something to make it more fitting for an evening of Hogwarts magic, so I made a most unauthentic Hogwarts badge, and crocheted a tie which was then mostly covered up by the jumper.
For the tie I crocheted a length of suitable stripes with a few rows of decreases and then increases in the middle, folded it in half lengthwise and stitched together so the seam is in the middle on the reverse and then made a small tube for the elastic - so really a crocheted version of this Debbie Bliss knitted tie. And just in case you need to know, the most Hogwarts-like colours I could find in the local wool shop are Debbie bliss baby cashmerino in sunshine and damson. Just in case you feel you need to make a Harry Potter tie...