Well, I think I'm getting to the bottom of it. After the recent Audi health check (
here) indicated that
all suspension bushes were worn, I've had them replaced all round.
A trustworthy garage I know first replaced them with third party parts and the rumbling under braking was still there (warped discs sensation). They then got in contact with a VW master tech who announced something I've never heard of before and would be staggered if it's true...
I've recently fitted the bigger 20" Cantona wheels from a D3. This tech believes that D2s which came from the factory with 20" wheels (FE cars, for example) had uprated and meatier lower arms in the suspension to accept the extra load. The lower arms which the garage fitted yesterday were rubbing on the hub at one of the ball ends so it's back today to be fitted all round with Audi OE arms this time.
If this doesn't cure it then it must be an issue with either the hub or brake discs not properly aligned, surely?