If you have one near by UNI-QLO do amazingly good, plain, and well made jeans for about £15-£20
I like Uni-Qlo but not a single pair of their Jeans, even the relaxed fit get over my cyclists thighs.
I've just bought some Howies jeans and at the fourth time of asking I've found a pair that actually fits ok, quite high waisted though.
Best Jeans I've bought in recent years for longevity and quality have been Energie jeans – fantastic denim.
Diesel very good too.
I'd swerve Fat Face – not greatly impressed with their stuff.