Was at a party the other day and met a girl who was hot and interesting. We had balls loads in common but she was hard work - so much so that I gave up after about five minutes but tried reengaging at various points in the evening.

She was really excitable, like a kid who had eaten too many sweets, far too talkative around her friends (no wingman was on hand to help me out), and well a bit weird really, she had lots of in jokes with one particular friend which made no sense to anyone else.

There was one moment when she wasn't like this near the end of the night when her friend went out for a cigarette and she suddenly went 'normal' and actually engaged like a sane calm person briefly, asking me loads of personal questions and throwing some iois (nothing horrendously strong but they were there), but as soon as her mate came back she reverted to type.

So my question is how exactly do you deal with a girl like this?

You answered your own question. Isolate. Get her away from her friends. It's a good sign that she is giddy and talking fast, that shows she is interested in you and trying to impress you. Whenever her friend is there, she will do all the in-jokes with her for 2 reasons: 1) No girl wants to look like a slut in front of their BFF and 2) you hadn't built up enough trust and rapport with her, so she didn't feel comfortable enough to be overtly flirtatious with you (yet).

Number close and day 2 her (alone). She will be a completely different person without her friend there judging her all the time.