Took a couple of attempts, but I now have functioning gauges and the faces are not flaking off! First I ordered a whole new instrument panel from an old XR3i, but it turns out the inner workings were changed completely around '88, so only the speedo was useable. I disassembled that and swapped my original odometer in to keep the mileage.
Luckily, it turns out that the fuel/temp gauge faces are nearly exactly the same for early Mk5 Escorts, so I was able to get one from a breaker's yard, and after some careful trimming it could be fitted.
Here are some pictures of the end result: