Having got the Motorola Atrix (~£15 pm on tmob) I would say it is a bargain. The phone is quick but she might not like the size though.
Knowing girls, she's going to want a better Camera than what is available on the Atrix.
I've seen the Desire S for £25 per month on a 12 month contract, 300m, 1k text, 1gb internet, was at e2save I think, plus the shutter speed is very fast from what I hear for taking quick photo's.
If you are willing to spend just a little more she could potentially get a sgs2?
Dunno if this link works:
http://www.dialaphone.co.uk/aw2/bas...ct&item=offer&action=add&offercode=24VFSL0HN0
£60 initially (but £50 cashback on quidco)
20.50 per month - vodafone
300 minutes / 250mb internet / unlimited texts
Could be good if it fits her requirements
What about a Galaxy W?
1 - Android or iOS - Android 2.3
2 - Decent size screen (3.7-4" screen, 800*480+ resolution) - 3.7" 800x480
3 - Not too bulky - 115.5 x 59.8 x 11.5 mm Not massive but a little thick maybe.
4 - Good "bang for your buck" in terms of monthly costs (sub £20?) - From £13 a month after cashback
was going to mention this one but wasn't sure if it was out, look to be a really good mid range phone with a nice camera and flash, Scorption @ 1.4ghz.
or if you fancied a smaller galaxy s2 you can always look at the Galaxy R
Xperia Arc or Arc S.
The Desire S and Galaxy W look like the current front runners, with the W looking more future proof with the faster processor.