Hi illcar.
Unfortunately the information you listed does not provide any meaningful information, such as the foam density in the mattress you currently have. Please read the information you need to know about a mattress in this article that I linked to in my earlier reply.
Generally speaking the major manufacturers and brands (such as Sealy/Stearns & Foster, Simmons, and Serta) all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors. Lower quality materials and components will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here. along with post #3 here. and post #12 here. and post #404 here).
Knowing that the foam density is usually lower in the major brands than what I would typically recommend, you may wish to consider a refund on this mattress, as I wouldn’t recommend keeping it or adding a topper on it.
If Sleepworld offers a product for which you can find out information listed here and the exact specifications (such as foam density) and the quality of the componentry in the mattress so that you can compare it to the durability guidelines here. you may wish to consider an exchange as long as you first test the product for PPP(Pressure Relief, Posture and alignment and Personal Preferences) to assess if it meets your personal needs and preferences, and make sure that the quality of the componentry meets the guidelines I linked to above.
To find out how to assess a mattress I would especially make sure you’ve read post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well he will sleep), durability (how long he will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Once you know the specifics of a mattress or construction you are considering fell free to post them here and I’ll do my best to assist you.
Phoenix