Yes we do (actually .usr.bak) - but looking this morning I think I have discovered the solution to my own question...
Original maintenance plan said 1 day - I changed this to 24 hrs - no effect (so presumably it keeps an underlying value)
I tried changing it to 48 hours - ie a real change - problem *seems* to be sorted.
I could change it back to 24 hours but I think I might leave well alone, unless we start running out of space!
Thanks for all your suggestions!
:o)
Jack