Dave
This sounds like a classic pressure switch failure to me as mentioned by John above. I assume the Series 1 Roadsters used the same switch as my 1999 +8? (Others like DaveW will know better than I)
Replacement of the switch itself is the only cure.
This is not exactly easy unless you can raise the car high enough to get good access from underneath. If you can, just remember to hold the switch body very firmly as you release each of the pipes , you don't want to bend/twist them as the job becomes a lot bigger if you do.
