UKH

Are they for real? I've had one for less than a week. Not only do I feel aches and pains more when I'm on it but also when I get up in the morning. (I also feel a couple of inches taller than I remember!). Can anyone report back with any long term benefits that will make this tortuous routine worthwhile?

It is a meaningless phrase really, generally it will mean the mattress is at the firmer end of the spectrum, but does not really mean anything. You can buy a mattress that says "orthopaedic" from £100 for a double.
A Hard mattress is not better for your back, in fact it can be worse, in that if it is too firm, it will not allow you to sink into it so that it can meld to you and give support.
If it is too firm, your best option would be a mattress topper, but I would not spend too much on one.
How much did you pay?

£424!

Should be a reasonable quality mattress for that price. Give it a another couple of weeks, and see if you get used to it, but the only way you will make it more comfortable is with either a Topper or a Sprung Base. A very common mistake buying a mattress thats too firm in the mistaken belief that it will be better for the persons Back.

I'm convinced that mattresses are marketed by the same people who market toothbrushes. All sorts of fancy features and reasons why they could make a transformative difference, but in reality they just make your gums bleed.

I can see me getting a topper at this rate, thanks for the advice.

if its sprung (pocket springs hopefully) you may well find it softens up a fair bit over the first few months of use. I get bad backs and personally get on best with fairly firm pocket sprung mattresses (my last few are the type with a foam top built in)

Buy a thin memory foam mattress topper!! I did the same as you describe about a year ago....firm mattress that made my hips and most joints stiff and ache every morning but now I really like it with the addition of the topper....a real game changer ..!

It may help the OP, if you say what you mean by "thin". I would suggest 50mm is ideal, with 20mm too thin, and 75mm as too thick making getting bedding awkward. How thick is yours?