Ok... the comments I would pick apart are :
They say that it was manufatured in 2003 and the problem was related to the cable which has since been rectified ? Obviously not, since a programme was made about GHD irons blowing up and it is
still happening. It is also a very common problem... as has been witnessed on here.
Furthermore, it should have been made to withstand daily usage anyway... I bought an iron (for clothes) from Tesco for £4.95 several years ago... use it loads... and that hasn't blown up in my face. Neither have any of my other electrical appliances, including hair dryers ! They are trying very hard to wriggle out of any responsibility here... by saying that it's to be expected.. wear & tear, etc..
They have also implied that it's
your fault by putting "undue strain" on the cable

. How exactly do you do that then ?

It's used to style hair... not to hang things from !
They are no longer able to support this type of iron as the parts are no longer available ? Duff then, were they ? Why else would you alter the components of a product ? How convenient....
They are taking advantage of the fact that you have no receipt in order to fob you off. Unfortunately, having no receipt doesn't help you... so all you can do is argue back and stand your ground. The fact they are now charging you for the return of the irons is appalling customer service ! If they are not built to last a reasonable amount of time... then they are hugely over-priced for their intended lifespan.
You need to get your irons back... but keep the pressure up. Ask them for clarification on the lifespan for a pair of these things... mention the CAG... that scores of others have had similar problems... and if they are not prepared to take the matter seriously, that you will gain the support of members on here to take things further and raise public awareness.
Have you seen Bookworm's letter on Page 1 of this thread ?