Looking at the 1km radar:

Main band of rain encroaches upon the western tip of Cornwall ~18:00 on the
16th, reaching St Austell Bay at ~20:00. Most of the rain rates are 5mm/hr,
but a few heavier regions ~10mm/hr.

After 21:00 a stronger line of rainfall (10m/hr) appears aligned NNW-SSE
through Penzance, moving eastwards through Falmouth ~22:00. Although this
line dissipated somewhat as it moved over St Austell, there was orographic
enhancement over Bodmin Moor over the next 2-3 hours. Meanwhile out to the
west a thin line (5km E-W) of very heavy rain (50mm/hr) developed. This
line first appears fragmented, but started to form a continuous line from
northern Cornwall, southwards across the Lizard and out into the channel at
~03:00 on the 17th. This line then develops slightly to the east of the
Lizard, so that by 04:00 it stretches from St Austell, south-westwards just
to the East of the Lizard. On the southeast side of this line (about 50km
out over the channel) there appears to a much drier region being dragged
northwards. The thin line of heavy rain then wriggles north-eastwards,
decaying over the next 3-4 hours.



Given the thinness of the heavy rainfall area I suspect it would have missed
many of the gauge locations: maximum rainfall accumulations based upon the
radar suggest in excess of 50mm in a band no more than 4-5km wide; 35 sq
kilometres with 75mm total over Bodmin Moor, and ~ 13 sq kilometres with
75mm on the St Austell area.