Hi Alistair,
The R31 region recently had a morning at Its Your Garage, where you rent a ramp to be able to access underneath your car. You rent by the hour, as most people dont have a ramp at home (there are others round the UK, this happened to be near me in Culham, Oxon) Anyway, you could do this yourself at somewhere like this, but they also said they have a local company that dont charge much, that steam cleans ad degreases the underside of any vehicle, for a fixed fee. You can (or they can) then add anti corrosion waxes etc, if you like, or, you can replace the corroded clips etc, hopefully easier to remove once all the gunk has been cleared away
As mentioned above, removing any corrosion is a good thing. There are also some things you can do at home. I have 2 adaptors for my pressure washer. 1 is a 45 degree angle nozzle for getting underneath to remove debris. The other is a 'trolley' that slides under the car to rinse the whole underneath. Most later Porsches like ours are protecetd by under trays though, so for me, I do go all the way inder, but the main usage is for the rear engine, exhaust etc, that isnt covered.
If you have anything like this, you could also use it to apply (if you had an intake for your pressure washer) Bilt Hamber anti corrosive products, other products are available!
https://www.bilthamber.com/corrosion-protection-and-rust-treatments
HTH
Mark
