Thank you for your replies guys.
Umh.. There are nice and convenient (and cheap too!) 16bit ADC modules built around ADS1115 IC. Unfortunately this IC does not have a Vref pin at all, but it can be used in differential mode.
For noise rejection and all that it's of benefit to use ADCs in differential mode in any case.
A precision voltage reference like a MAX6126 with 4.096V output and 0.02% error will do nicely for the 'top' or upper input of a differential setup.
As for resistors and temperature drift - that's a good point. Just checked with local supplier (RS components), they do stock many variants with 15ppm/°C temperature drift at fairly low prices (under £2 per bad of 10).