Citroen C3 - you can get a low(ish) mileage Mk1 for silly money.
I ran a 1.4HDi 8v for 10 years / 162k and it cost me virtually nowt outside routine maintenance and consumable parts.
Sexy, eh? Note that funky double glove box arrangement (the lower one is tiny though...) and the really poorly positioned interior door grab handles
I've had two bikes in the back with both wheels on and both front seats occupied (rear seats folded) - a
very tight squeeze but just possible. Take the front wheels off the bikes and it's easy.
Petrol might be better for you if you're looking for maximum cheapness. Possibly avoid the underpowered 1.1, but the 1.4 is a reliable lump and adequate for most purposes, the 1.6 marginally quicker but thirstier.
The flappy paddle auto box seems to be universally hated in reviews, so manual might be the better option.
The early Mk2's (2010>) are starting to look like a real bargain too.