Loads of ideas, loads of choice and decent wedge of cash. Perfect
First off age.
If it's a 'more reliable' brand such as Toyota a year or two old with low miles will be perfect.
If it's French and you definitely want reliability by ex-demo if possible with a stupendous discount. For the 107/C1 this maybe less critical because Toyota are involved at the build stage. I happen to trust Renault a little more so 1yr old should be fine.
Fact is everyone here's right about value, 3-5yr old cars are amazing value, but if you feel it's a risk you want to avoid fair enough.
Small and sensible:
Well you already drive a Yaris so I guess you want a change? If you're still interested a T-Spirit or SR would meet all your needs without being boring but you'll have to compromise on age so the very last cars of the previous generation might be ideal.
Aygo/107/C1 are great cars but a Yaris will be better over the miles and seat 3 in much more comfort. I'd be looking elsewhere.
Like the Yaris a Jazz would be great but age is again a problem. If a Fiesta is on the list however I'd still be tempted to go a year older and buy Japanese.
If you're thinking VW-a-like the Polo/Ibiza/Fabia are all good cars if a little bland. That said the big diesels are easy to insure, relaxing to drive and have alot of poke for a small (if a little heavy) supermini. As someone else pointed a Fabia vRS would be the default choice but it looses out on age vs budget. A VW Fox may be worth a punt if you're after really new, Top Gear think it's decent football player at least!
Middling
Fact is the Focus/Golf/Auris/Astra/Civic etc are way too expensive at a year old or less even with the most basic models unless a dealer offers a show-stopping deal. And because you'll want proper warranty if it's secondhand you'll be at the mercy of a dealer.
However a car supermarket may help you out here as the best ones have ever-rotating stock and loads of choice. Just remember, when you're tempted by the 1yr old Focus TDCi with 46k miles on the clock, manufacturers warranties are limited by mileage as well as age and most won't care if it's still under 3yrs old if you've just rolled past 80k. Hyundai/Kai may give you 5 or 7 yrs however.
IF you really fancy a 'normal' hatchback then you may find an early used Kia C'eed in your price range. I know, I know you said no Kia but it's big and modern with loads of warranty and comes with good , if not raving, reviews. Depreciation will be the biggest hurdle but it ticks many other boxes!
Left-field?
Suzuki Ignis Sport - They appear to be made of extremely stern stuff (see other thread), look great and are cheap to run. Would make an interesting Yaris comparison.
Fiat Panda 100HP - Buy less than a year old or ex-demo and you should be trouble free. Superb car.
VW Lupo 1.4 TDi Sport - The newest (05) are now nearly 3 BUT a great little car if the third person you mention has no knees.
Ford SportKa - It's nippy and cheap. Not as good as a modern Fiesta but it'll be faster for the cash! I'd still buy a Yaris though.
Cars like the Corsa should be available from a Car Supermarket or somewhere like Broadspeed but I suspect it's too new in reality.
One quick note:
A couple of the cars mentioned here are relatively new to the market. I would always be cautious if you're buying a car from the first 6 months of a production run of a new car. If the car survives 5yrs+ with a clean bill of health then it'll probably be decent all its life. However if none of the cars available is more than 6 months old tread with care.
Facelifts don't really count as the majority of the car is a known quantity.
As often is the case my appologies for rambling. I'm still pondering a decent left-field car as I agree with the peeps about doing something different, the challenge is to get everythingelse as well!
Oh yeah and if it were me the Yaris, Jazz and Panda would be at the top of my test drive list