Thanks for all replies - as luck would have it, a ferret in our garage turned up some old brakes that I kept from the last Kuschall - with working spring. In a twist that does make me think I should buy a lottery ticket, it's the same part.
Not that this lets Wheelchair Services off the hook. Although the bottom line is if they can't get a part, they can't get a part. At last SWMBO has a wheelchair and a partner to help. Apparently there are some people having to go into residential accommodation as they can't be provided for.
With the way Omicron is going, that must reduce the odds of surviving the next few months a bit :(