Miele KF7542 sealant melted!

I have a 5 year old Miele KF7542 SN No Frost fridge/freezer which every year, in the height of summer, struggles to keep down to -18C.

Usually I defrost the evaporator and leave it at that but this time I decided to check after a week to see how it was getting on. I noticed that ice was starting to buid up again at the top right hand corner of the freezer box, where the tubing passes through from the outside to the inside. I looked round the back of the freezer and found that in the same area there had been a sticky grey sealant which seems to have melted and dripped down the back of the heat sink towards the compressor.

It seems to me that warm, moist air is able to pass from the room to the freezer section becasue this sealant is no longer in place to do its job. Is it possible for me to replace this sealant myself or is it an engineer callout? I would have to remove the heat sink to get at the area I think, and this covers the whole of the back of the fridge/freezer. I would be concerned about damaging the pipework if it is rigid, but if I can move it back by an inch or so without causing a problem I would probably have enough working room.

Finally, any ideas what sealant I should use in this instance?

Many thanks

Neil