It was raining and I was cold, but I had a little look around Datchet and Windsor to see the parking situation. I didn't check out Old Windsor, but I am guessing we don't want to block up the streets and piss off the residents. We can piss off Datchetites as much as we like.
The parks in Windsor are Pay'n'Display, and £6.50 for all day 7am-midnight. I didn't start putting coins in, so I don't know what would happen if you put in £13 - would you get a ticket for 2 days? However, the pay'n'display uses
http://www.parkmobile.co.uk, which from reading that site means that you register then use an app or send a text, and they will charge your credit card and electronically tell the attendants you have paid. So you could pay&display on saturday, and on Sunday pay again via phone. This Saturday morning there were easily a hundred empty spots a little before 11 in the Edward VII carpark.
But much simpler would be just to park somewhere in Datchet. Close to the river it's all single and double yellow lines, and residents permits only, but I approached from the A4 on the B470, there are lots of suburban-ish streets, with no apparent restrictions. For example,
this quiet road running beside the B470 looks nice and safe, and there were the same sort of numbers of cars parked there this morning.