UKC

My gaiters always seem to fail with the underfoot strap. Either one side or another rips off, or (as in my latest example) the 'tooth' in the buckle gets too bent to work on the belt. 

Does anyone know a repair company that might repair this kind of heavy duty strap? 

Lancashire Sports Repairs seem to be your people

https://lancashiresportsrepairs.co.uk/gaiter-repairs/

In my local market there is a genuine "old school" cobbler. I buy my new strap from the pet shop, dog collar/lead, and then he stitches it on. 

Any old cobblers near you?

Piece of wire coat hanger cut and bent into shape has always provided a permanent solution for me

For gaiters with simple straps (no velcro, no buckles etc) i.e. just the ones that thread through a plastic keeper I use furniture upholstery rubber webbing to replace the straps and just chop a strip to size.

I bought 3m of 2 inch wide Pirelli Rubber Upholstery webbing back in 2013 for £12.50, (it's over £20 now) and still have at least half of it! You can cut it to the exact width and size needed and it's much tougher than the replacement straps you can buy. 

For a repair, try a cobbler or a clothing repair place. My local repair place managed to resurrect a rucksack multiple times with various straps etc being replaced, re-enforced etc. I've also had them fit heavy duty poppers to gaiters and waterproofs when the ones they came with fell to bits. 

Wire cable is much better, not in stock but may be available elsewhere.

https://www.glisshop.co.uk/gaiters/tsl/cable-for-gaiters-high-trek-black409...

When I failed to find anyone local who could reattach straps I pop rivetted them. Has held for several years now.

Another vote for a good local cobbler. You basically just need someone with a heavy duty sewing machine. The chap in my local town charges me about £3 to do this sort of thing. 

I got mine replaced with some of those light weight load straps. The pull tight kind with the spring loaded knurled roller, (not the winch type!)
You can even have them stitched on so they go over the top, with the webbing continuous rather than cutting it, so then the stitching only holds the gaiter in place rather than taking the force of pulling away as you walk. They should make gaiters like that to begin with tbh..

Paracord does the job just fine.

Personally I've always opted for neoprene straps with "toothless" buckles. They're pretty much immune to snow and ice clumping in winter, and they invariably outlive the gaiter to which they are firmly attached. 

Me too. I just cut them off and manage without.

I bought some nylon webbing of the correct width. Far better than the self destructing Paramo rubber straps.

I always found threading the strap through a piece of tubular tape limited the damage (or at least delays it).

Tying on a piece of single core insulated electrical flex works. Normal PVC insulation works but is not as durable as XLPE or PTFE insulated which unfortunately may not remain knotted as well.