Discovered that the odometer reading on the Garmin display had reverted to zero (it is on 'long term') & think it must have happened when I changed the clock on the dashboard display by pressing the RH button. Feasible, & presumably unreturnable? Thanks.

the long term figures on the PID relate to memory 2 on the trip computer on the dash

 

the short term figures on the PID relate to memory 1 on the trip computer on the dash

 

resetting either of these (via the right hand stalk) will not be recoverable and reset the figures shown on the PID

 

as the PID just reads information from the ECU

Many thanks - that answers my question. Dennis.