Situation almost resolved.
Without me having to do anything other than making a polite phonecall, Porsche UK have kindly agreed to supply the damaged parts - ECU & alarm unit & 2 new keys. I'll pay for OPC labour, £199 approx. Porsche stress that this is a goodwill gesture and if the problem reoccurs it'll be my problem next time. I'll take them up on this offer but I'm still not entirely happy, but life is to short to push it any further. Their customer service people told me that clearing the drains is a regular maintenance task to be carried out by the dealer during a service, however my dealer had told me it's not on their checklist, so where does that leave us? My car was serviced 2 months ago so maybe if they had checked it, this wouldn't have happened, so I believe my having to cough up £200 is not exactly a fair result but I'll cut my losses with the £500 saved on parts.
Porsche UK don't seem inclined to do anything that might prevent the problem. They say the dealer is supposed to inform the owner that it's a maintenance task, but my OPC says this is the first they've heard of it. As it stands these drain channels can block whether or not you do a visual check under the 'clam shell' lid. I was in the habit of checking it every so often but, without a strainer at the top of the channels, unless you also go to the trouble of pouring water into it on a regular basis, you'll have no real idea what state the drain is in.
So on a regular basis (I'll be doing it each time I wash the car) you'll need to open the soft-top half way and actually prove to yourself that water is going down these drains. Removing any debris you can see is not sufficient, you'll need to put some water in there and check it comes out underneath. It's obvious when you're told about it but how many of us have been told or how many owners have even bothered to see what's under the 'clam shell'. It's not in the manual so IMO Porsche don't have a legal leg to stand on.
I also have to say that this forum should be the first port of call for any owner with a problem, and the OPCs could get a lot of useful advice by logging on too
I promise I will finally get my act together and sign up as a member.