At the recent Dunstable Downs rally I picked up a small LCR bridge at I price I could afford to lose. It seems to sort of work; the basic bridge components are 1%, so having 3 decades of resistors seems optimistic.
I've had a closer look now. Looks like the sensitivity pot is excessively noisy. The resistance range isn't bad, but the capacitance range is awful: 500nF reads as 437nF, 40nF reads as 35.4nF. Since they are mica reference capacitors, I know which I trust. Note that both of those readings are c12% low. Hmm.
Opening the box reveals a cramped interior where the PCB is "glued" to the front panel by all the switches; prefer not to fiddle with that. But on one edge I see a solitary large capacitor, 0.1µF 600V without a tolerance. Hmm. Measure that (in circuit) and it is 113nF

Bodge an ~87nF capacitor in parallel and 500nF reads as 203nF. Bingo.
Started looking at precision 100nF caps on fleabay. There are some, but they cost £15-20, more than I paid for the bridge. Not going to happen.
Eventually realise I have a component drawer full of polystyrene caps, and that it wouldn't take too many to increase that 87nF cap to 100nF.
So tomorrow's job will be to fettle with capacitors to make a 500nF reference capacitor read as 500nF.