Putting on the power

Not all forks lock the same way. Some do lock solid, some leave a short bit of (stiffer) travel. You need to figure out which yours is.
Then sus forks are just about never as stiff as rigid forks. Even if they lock solid. The usual design is one leg for spring and one for damping - and lock only on one leg. They will try to move separately. Rarely noticed while JRA. Sharp turns, hard braking is generally when fork flex becomes noticeable.
Also, consider the tires, particularly if you're heavy.
MTB tires are about twice as tall and maybe 4 times the volume of a road bike tire. On a proportionally narrower rim. And at maybe half or 1/3 the pressure.
I'd look at all those before looking at the frame.

Yep only one leg locks, but not going fast enough yet for a problem on turns or braking tbh. lol

I'll up the pressures a bit and try that out. currently at 35F/40R PSI, this gives only very minor "distortion" (for want of the correct terminology) when I am on the bike. I'll up it to 45/50 and see if that improves the feeling.

as per the OP it has been a long time for me between riding road bikes to now MTB, so I also have to admit that I am simply forgetting the action. I am also a bit paranoid about whether I have bought a bike strong enough. No reason to believe I haven't of course, just paranoid.

MAny thanks Dabac, once again, for your answer to my questions.

While not impossible, it is quite rare for bikes to break while JRA - even under heavy riders.
For a MTB to break during road use is doubly rare.
Some things to consider:
- the more you pay, the less you get. Meaning as bikes get more expensive, they also get lighter. So don't expect the most expensive bike to be the strongest one. Don't expect the cheapest one to be the strongest either. The strongest bikes are probably found somewhere mid-range. Expensive enough to be well engineered and well built. But not expensive enough to have the thinnest tubes.
Aluminium frames are known to crack every now and then. But very rarely in an immediately catastrophic way.
Usual places are headset bearing seat, bearings for rear swings and seat post cluster.

dabac said:

seat post cluster.

You called it right there. lol

On a 2 month old Chinese frame (yes I know Chinese frame. LOL). Cracked right around the top tube just before the weld in to the seat cluster. Noticed it today on a slow ride in the city looking at bike shops.

Purely by chance I was able to buy a Specialized M Works MTB frame in the right size today and have everything put on it from the other frame.

Contacting the LBS tomorrow about the warranty. To be perfectly honest though I think I will be LUCKY if they even offer a repair. refunds and replacements of ANYTHING here are unusual. It will probably end up as a right off but a valuable lesson learned.