Dear Alex - Reg's description is correct and you are also correct in your location but you have to remove the grill at the rear if the fridge freezer to gain entrance to the box Reg is talking about.
I too had all these problems until I found this link and done some digging. In the end I repaired my fridge for around £25 and it was all down to the helpful people who gave there advice freely for all of us frustrated freezer folk!
So in return I have included the whole process I carried out with links to pictures and spare parts to help you. It's a bit long winded but hopefully it alleys some fears you may have when carrying out the repair.
The link that shows pictures of what is required were originally posted by a guy called Matt (Big thanks to you by the way!) which I found really useful when reading the other directions in this forum. Another big thanks to Gary at Advanced Cooling Systems co.Durham 07888827985, (December entry), very helpful).
The picture link is - picasaweb.google.co.uk/mrfreezer5/Repairs#
Just copy and paste into your web browser and three pictures will appear -
the first picture is in the actual freezer compartment (with the front panels removed) showing the sensor head.
The second and third are round the back with the grill and grey plastic cover (held on with 3 screws) removed.
Don't be spooked at removing the grill, I found that when I removed all the screws round the side, I was able to let the grill fall forward about 45 degrees without putting to much stress on the pipework at the bottom. But be careful, this is still a dodgy part of the repair because if you break/split a pipe then its all over. Just have some furniture at hand for support. A chair/table/partner will do the trick, (I was able to use a kitchen drawer), however I did find the pipes surprisingly sturdy.
Once you have removed the grill and the grey plastic cover with 3 screws, you will find a white holder (not connected to anything) holding the Printed Circuit board (picture 2, with the white cover removed)
Now you need to remove the rubber grommet holding the wires going into the back of the freezer compartment (picture 3). This comes out with a little persuasion with a screwdriver, but don't fight with it to much as to damage the cables it is holding.
From inside the freezer, remove the cable tie thats holding the sensor onto the pipe (picture 1), remove the putty sealing the cables to the back of the unit and carefully pull the sensor out the front to determine the connection you need to remove from the printed circuit board around the back. The connection is a simple squeeze and pull clip.
Once removed replace the new sensor just as you removed the old one.
The sensor i bought was from Argos spares - SERVIS temperature probe, no frost SER502023500 , web link -
argosspares.co.uk/cgi-bin/product.pl?PID=1117601&brand=&model=SER502023500&part=
The putty goes back in to insulate the cables and the rubber grommet pushes back in quite easily as long as you take care of the wires it is holding.
Basically that's it! refit all parts as you took them off and bob's your uncle! Hopefully a working fridge. For a £25 part and a Sunday afternoon I now have a working fridge with no annoying "H" and sour milk!
I realise other people have had other issues, but with reading this forum link I think the sensor repair will help around 90% of you out there.
Good luck
Stephen from Edinburgh, August 2009