An earlier train isn't an option (it costs £f*ckmehowmuch ?).
I have both maps, a GPS track and I'll study the route lots before Friday.
We'll get there as soon as we can

.
Copied over from my earlier post.
Exit the station from platform one/ticket office side, go to the end of the road, where you'll want to go right. That junction is actually left turn only, so use the cycle path on the bridge & the ped crossing there to get going the right way. Straight on from there until Kingston Crescent junction (streetview
here), where you make a left. Bear left at the
next junction, there's an underpass and a couple of crossings that will leave you right in front of the ferry port entrance. It's a couple of miles, so I'd allow more time than fifteen minutes if you can.
If in doubt, give me a ring and I'll lead you down. Same applies to
everyone, of course.