UK Auto Repair Questions?
Hopefully i can help.
One place i would maybe check is the scuttle,pollen filter area not to uncommon for this to happen on the vectra/signum models when you have the bonnet open just under the windscreen you should see plastic flap.you need pull part of the rubber trim away then lift flap and you'll see your pollen filter - right in at the bottom you see the drains or rubber gromets these sort of tapper at the end and usually always block up with dirt,small leavesand if blocked you will see lots of water still in there and cause rain water to back up when raining and spill in via the pollen filter and thus finds it way into the cabin or floor wells -I see you haven't mentioned that so maybe have look there and use long straight bit of metal rod and poke down in the centre of the grommet to try and clear it out .you can remove them if require there should be two check..next check is just to rule out any possible leaking heater matrix so check your coolant level hasn't dropped if it hasnt then the matrix should be ok....another likely cause could be door membrane is leaking. this is like a sheet plastic liner bonded behind the door panel, doors will always have some rain water run inside them getting in via the rubber seal around the glass and should drain away via the drain holes at the bottom of the door, if this membrane or sealant fails or has been damaged due to previous repair then water can come through in to the car from behind the interior door trim .to check those the door panel will need to come off so again something to bear in mind - also check the drain holes under the door are clear too..
One test you can try which i do see you have tried already is the water hose test after drying/partially to cover the whole area floor with newspaper/talcum powder around that area can help too highlight etc as any leak into the car will be spotted on the paper etc and give you rough idea as to direction the leak is coming from or coming in even.then with all windows and doors shut and with somebody else inside to monitor pour water either with a garden hose in specific areas - checking one area at a time starting from the bottom up in regards XXXXX XXXXX door seems / the lower edge of the both windows , windscreen for possible sealant fault / then all the way up towards the roof line then around the boot and rear light cluster unit until of course you do spot the water or drips coming in.. and see what you find,the problem with such faults as this is - they can be quite time consuming to trace down and sometimes may involve having to remove inner trim/carpet sections etc to trace it down.But check the door seals first both side of the car aswell as the boot seal too first for any obvious tears or damage/cracking or twisting and make sure they are fully seated properly also that drain scuttle area too under the windscreen.I do hope this helps..