Thanks, Chris/Oldbuffer, I didn't think they could be sourced, seeing as Bristan said they couldn't do it. My old waste leaked from where the washers are located, and the rod snapped where it lifts the plug, so I bought what I think is a new Bristan waste assembly from a private seller on eBay for £18.00, where the seller had only replaced the tap and the waste was going spare.
There is a rubber seat washer in the new waste (red arrow closest to the waste), but no washer for the other side of the ball. The ball fitted on the new rod is 12 mm in diameter, which is smaller than my broken rod, and the washers don't fit the new rod. Was wondering if this could be a pattern spare.
Found some new replacement rods and washers on eBay, will have to check the size will fit.