My seat harness had started to tear itself apart in various places, mainly the bottom edge of the 'Hard shell' material and the leg strap attachment to the same area as in pics.

In Perth after taking it to my local key / shoe repair place, he advised me to take it to 'Mister Mint' in Karinyup Shopping Centre, because they have all the Heavy Duty Leather shoe & boot repair machines..
The guy was unsure as it was the first time he'd seen one and left it with him.
Picked it up and he said it was easily sewn by their machines - no problems at all.
I thought the leg strap under the hard material would be a problem - but wasn't..

So if you live in other City's - try asking your local mister Mint which shop has the heavy duty gear and save some Coin...

Good tip, but I found with my old ProLimit seat harness that the loosely woven material used in the outer covering pretty much falls apart, pulling away from seams and allowing plenty of sand inside the harness. So I stripped all the useless material away and replaced it with seat belt webbing (easily sewn together with domestic sewing machine), hand stitched to body of harness and straps using a sewing awl (using the original holes in that hard material) and hid the whole unglamorous lot by adding some board-shorts to the outside.
That harness was a good fit but poorly constructed, and we didn't have a local shop at the time to shop for a new one.
I hope the cost of repair to yours will be well justified by the rest of the harness not falling to bits like mine did.