On and off over the past few years I've noticed a sloshing sound from the rear of the car and I assumed it to be from the fuel tank, but now I know it to be coming from the rear offside door.

I've not had time to look yet, but are their drain points to unblock?

There are drain points and if you have water in the door then one would assume they are blocked, so yes.

To follow on from that, they usually drain when the door is open so if you don't open them frequently, you may need to be more vigilant in making sure you do so, particularly after a heavy rain fall or a car wash, etc. Once open, you should see any excess water drain from the underside of the door in question. If not, have a close p look and make sure the holes are clear. 

This happened to me last year ,pull the door seals away under which ever door is full and poke the drain holes ,i must have had a couple of litres in there ,then give under doors and seals a good clean because they get quite dirty ,then hey presto bobs yer uncle all good 👍

There was a thread a while back about pulling these seals forward and cutting notches out of them in line with the holes, in order to allow the drain holes to let the water straight out. I did this when I got my Octavia 3 in May 2014 and never had any issues in the three years I had it.

They must be a little different on my Mk2, looking at Mickvrs's post. Mine are underneath, clear of the seals. ie, you don't need to pull the seals away to poke the holes.

 

So Skoda decided to put the drain holes for the door behind the door seals on the Mk3, therefore blocking them. If this is the case............Simply Clever.

Cheers Chaps, just pulled the seal at the bottom of the door slightly and let at least three pints of water out..........