For the BME280, if you use the standard library and setup it uses "forced mode" with a 1sec sleep between samples. This uses very low power, a few mico amps.

I found the problem. In an unrelated part of the sketch I was doing an analogue read from an input set up as a digital output. This apeared to generate some sort of error state that fouled up the interface between teh Arduino and the RFM69. After removing the offending statements everything works fine.