Good Afternoon Heather, 
 
Thank you for your question. 
 
You would probably be best looking at our Pocket Sprung mattress ranges as these will give a level of support from inside the mattress itself with the fillings sitting atop the springs providing the comfort layers. 
 
At 7st that person would need a Soft spring tension; This will generally cover anyone up to 8.5/9st. 
 
Once that is established it then the focus moves to the fillings as this is what gives the mattress it's "feel". 
 
With scoliosis we'd advise to avoid anything too soft as this tends to only exacerbate the issues. As such I'd advise to go for something more medium overall. Have a look at our Origins Pocket 1500 or our Artisan Naturals. 

The Origins Pocket 1500 is a true medium in the top upholstery providing a slight sink but not too much. This range is constructed with synthetic fibres. My concern here would be that this mattress comes with a medium spring tension as standard so might be slightly too much on the firm side for that body weight. 
 
The Artisan Naturals is a true medium in the top upholstery also but this range has the benefit of being constructed using natural fibres. The biggest benefits of natural fibres are that they provide a cool sleep surface making them ideal for anyone who has the tendency to get hot during sleep. Longevity also comes into it as the natural fillings have longer lifespans. With the Artisan Naturals you can specify a Soft spring tension ideal for the 7st body weight. 
 
Please see below for a bit more information on body weight guidelines;
 

 
We would always also advise that it's safer to go for something that's more medium overall as if, after 4-6 weeks, the mattress is found to be too firm then you can introduce a mattress topper to soften the feel down. You can't do the opposite however and firm up a mattress that's too soft so if in doubt always go medium. 
 
I hope this has answered your questions fully but if you do have any further queries please don't hesitate to contact us on 0161 437 4419.
 
Kind Regards, Lee.