Hi Chris
I'd go less heat for a longer period of time.
I recently did a stew for about 40.
Cooked for about 3 hours -
The meat was very tender.
Made the mistake of putting in veges at the start.
They turned to mush and became like a thick gravy and retained none of the texture or individual flavours.
Next time I will put them in an hour before the end.


Aart

I never bother with heat on top for a stew, and usually brown up meat and a heap of sliced onion before adding the other ingredients, Ive often cooked them at a slow simmer, stirring occasionally for 3-4 hours, never had the veg fall apart, Dunno how you managed that Aarty

I too brown my meat first - usually after I've tossed it in a freezer bag with seasoned flour. 
I think this helps thicken the stew too

Then the sweating of the onions, celery, garlic etc

These I do at medium heat from the bottom only. 
Usually adding the *pre-heated* water to the 'browning' heated oven is enough to bring it down to a gentle heat

Add the meat again, the stock/water, along with the potatoes and carrots cut into large chunks
Maintain just enough heat underneath to keep it just simmering - a bit of steam and very gentle bubble

Once the potatoes are half done I throw in any other vege I plan to use. Peas, corn, capsicum, cabbage etc

Near the end, once the potatoes are cooked, pour in a little bit of cornflour mixed with cold water to thicken things up, and maybe a dribble of parisienne essence to darken it if necessary.

I don't know about anyone else but I find a stew (or casserole) always seems to taste better the next day...