Welcome John.
If you're from the Western Isles, then your first job should be to Lanoguard the underside of your A3. I've got family from that part of the world too, so know all about the salt-air, and the destruction that it will ultimately do to your car, never mind the local council salt and gritting the roads.
Wash the car as normal. Remove each wheel and jet-wash under arches, etc. If you want to be thorough, then remove the inner wheel arch liners too and make sure you remove soil and other road detritus which will be lurking in the base of the wings. Let the car dry. For any crusty areas underneath on the subframe - you can use a wire brush. Spray Lanoguard (basically lanolin) on all exposed metalwork except the exhaust and watch going near the brakes.
A decent dry sunny day is perfect. Leave for a few hours to harden. Repeat annually. If you're doing it on your drive, or lock-block or such like, stick down a tarp first. If it's bare ground - crack on.