If you don't have any luck in Tesco, ASDA keep it, it's usually with the pudding/dessert mixes.
Durum wheat is one of the hardest wheats and has a high protein content which produces the gluten that gives the dough it's elasticity.
It's more expensive than common wheat, and if it doesn't say durum wheat on the packaging, then it probably isn't, being instead a hard flour ground from common wheat.
The softer/weaker flours, such as 'plain flour', have less protein in them.
Flour produced from hard wheat tends to be more free flowing than flour milled from soft wheat, because the hard wheat grinds to a more granular texture - so it will pour in a way that soft/weak flour doesn't (rather like the difference between granular sugar and icing sugar).
Hard flours are used for bread dough, pizza dough, pasta, and some pastries, such as puff.
Weak flours are used for cakes, some pastries, etc.
You haven't caused an argument – I'm always happy to agree to disagree.