A Southern Water contractor at the small group of lowest lying houses
this morning in St Denys , Southampton, sorting out their sewage
problems from last night, the main group of slightly higher level houses
escaped this one.

From previous measurements I reckoned about 12 minutes of 180mm
average/hr rain or 36 minutes of 60 average mm/hr rain would reach house
floor levels, ignoring air pressure imbalance issues.
Totally ignoring the 20:00 rain as plenty of time for the Archimedian
screws of the local sewage works to handle that, back to normal levels.
From the local retired professional metman's rainfall bucket gauge I
reckon graphically from about 22:15 something like 20 mins of 55 average
mm/hr, 4 min of 90 av mm/hr, 9min of 150mm av /hr,4 min 130 av mm/hr
,4min of 75 av mm/hr.
The 20 minutes of rain would have pre-charged the system and then
dropped back somewhat during the lull about 22:40, no idea how to allow
for that.
But 9mins of 150mm av/hr on its own probably means about 2/3 of the
headroom available before residential floor level reached.