Thank you everyone for taking the time to reply.
I note that Rotties keep coming up and they were indeed my first choice initially.
My partners family has had Rotties over the years but what's put me off is the health problems I keep hearing about.
Their current dog also gets quite knocked around by the heat up here and I'm really trying to avoid thick/long coats because of this reason (and the vacuuming).
This led me to look at more durable dogs that do ok in hotter climates, Ridgebacks and Cattle dogs seem to standout in these area's.
I really like the Ridgebacks but the temperament I've seen in the ones I've known has been as Dave described above: stubborn, independent and harder to train.
What I have always loved about my Amstaff was her obedience and eagerness to please. Chalk and cheese compared to the naughty JRT who continually pushes limits of what he can get away with.
Why not another Amstaff?? A few reasons, my girl has never really shown any inclination to guard the property, she had a lot of skin irritations (common for Staffy's) up here. Also she was a fairly dominant at times around some other dogs and once got herself into a fight whilst doing off lease agility.
Mastiffs...I very much like what I read about their temperament but I'm not really a big fan of the facial features of the bull mastiff (Sorry mastiff lovers).
I was also warned by a vet about dogs with rolls or folds of skin because of potential skin problems here in the tropics.
I visited a Cane corso breeder and liked the look of their dogs compared to the bull mastiffs I have seen around, very athletic bodies, more pronounced snout and less jowly in the face. They are still certainly a breed I am considering.
I guess the other option is to go for a rescue, the shelters up here a full of Bull Arab,Mastiff X type dogs (similar to the dog you mentioned Simply Grand).
Pigging is a popular pastime up here and its reflected in the breeds that end up homeless unfortunately.
I would rather a puppy though so my son can get the full experience of caring for, raising and training a dog through to adulthood.
It's just the lottery of temperament, size, health that makes me reluctant to go this way.
I just thought I would put it out there in case there was a breed I was missing.
Looks like I've got a bit more research to do, that's ok though because it's not something I will rush :)