I would not trust a Vauxhall dealer in Plymouth.

A family member (other halfs brother) lives down there and he brings his Vauxhalls to me for fixing as I work @ a Vauxhall main dealer as well.

They have charged him fortunes before for problems which do not get fixed, even writing off an engine ECU for a bad earth problem.

As said above, have a closer look.

If need be, use some talcum powder on the inside of the bulkhead panel (inside the car) and then repeat the water test. Any water will show up as a stain on the powder.