Hi Mary,
 

I have viewed the details on Benson's site for the model you refer to and it appears to be what is know as a "pillow top" mattress, which we would never recommend mainly because once the top fails, which it will do over time, the whole mattress needs replacing. However, if you buy a mattress with a separate topper, the topper can be replaced when required at a fraction of the cost of a new mattress.
 
I cannot comment on whether the mattress is good value for money as the description provided is vague in the extreme with no specific details of the upholstery layers provided. I am also unable to advise as to whether this mattress is correct for your body weight as there is no indication as to what "comfort level 2" is in Bensons. I would say that 75cm for a combined height of base and mattress is slightly high and in your case may not be suitable for an attic room. Are you able to cancel your order or opt for a lower combination?.
 
Kind regards Mike.