Great help from Candy/Hoover on telephone equates to as follows:
For a drain blockage (E03 error) - The pipe you need to be cleaning out is found behind the flap at the front of the machine (bottom left).
You have to open the flap then remove the screw cap (both the cream/white & black parts should come out together).
Then feel inside the cavity with hand for the draining hole (should be to the right).
I cleaned the hole using a hooked coat hanger and it fixed the problem.
I'm sure there are proper tools and be careful using anything other than a proper tool - I hooked my coat hanger tightly into a reed shape so no 'prongy' bits were able to damage the piping.
If you have some leaking afterward, you may not have screwed the screw cap back on correctly - it's a little awkward.
I had washed a rug and the back had disintegrated into powder which had blocked the drainage tube.