@DevonPaul where in Devon? I grew up in Torquay and all cars rust on the south devon coast because of the humidity and salinity in the air is very high. A good garage provides a degree of protection but its particularly aggressive in that area.
This was in Plymouth near the station, before I moved Up North to Exeter.
There was a DPM under the base, as the side door was quite short with about 2 feet above it. When I replaced it I thought it easier to trim the base down a bit to get more head height and found the DPM an inch or so under the floor.