Aeros start at £2.5K for a petrol with non-stellar miles.
EDIT: I stand corrected. There are a few about from £1400. There are only 2 on autotrader for less than £2500 with less than 120K on the clock. (Dan's £750 gumtree example is a poor high-miler that will need work so I wouldn't include that).
£2500-£5000 will buy you a nice one with less than 100K on the clock.