Well, you may but that doesn't mean the SGA covers you for that time.
The SGA would have been the legislation in force at that time of purchase and the presumptions in your favour cover you for six months.
Thereafter, you are still within the period of limitation but the burden of proof is yours to show that this propensity to have this fault existed at the time of sale. Obviously it would not have had that particular fault and if it had you would not have bought it.
The problem with 'breaks' is that they can be caused in many different ways. One of them can be because the product is weak although one might have expected that to arise before 18 months. Another can be just that there has been an accident.
Either way, the SGA rights are against the seller.
If you were to sue they may well just throw their hand in and concede as they won't want to spend time defending this.
I'm afraid you are not entitled to a full refund now though unless they rolled over entirely. You have had 18 months of use from the bed and that would reflect in the refund.
I'm very sorry but that is the reality of your situation.
Can I clarify anything for you?