Cheap chair for a tiny woman

[Salty22]

I need to get my wife an office style chair for our double desk, currently shes on one of the dining room tables and its not doing her back many favours.
Shes had back problems in the past (dodgy L5S1 disc) so if there any kinda chair that'll support her back well that would be good.
Shes 5ft and 55kg so doesn't want something thatll swamp her.
Got an upper limit of about £100, any recommendations guys?

 

[Salty22]

Sods law a part of the bottom of my own crappy old chair has broken this morning, tempted to pull the trigger on two Noblechair Icons lol
look at the state of this, its only 3-4 years old too.

 

[Salty22]

Any examples mate?