Hotpoint FFU3DW – Element at back of freezer keeps icing up

As above really – I have a 4-year-old Hotpoint FFU3DW which recently started icing up over the coil at the back of the freezer compartment, with the result that the freezer and fridge stop cooling and the alarm goes off.

If I manually defrost the coil (using a hairdryer or steam cleaner, etc.) then the problem goes away for a couple of weeks then back to square one. From a little reading, it seems likely that the defrost function is failing due to a problem with one of the following:
– defrost heater element/fuse
– defrost thermostat
– defrost timer

Is there a straightforward way to find out which component has failed so I can buy and fit a replacement? I do have a multimeter but I’ve never really used it in anger! I have no idea how to apply that to components and whether it is possible to do it with everything in situ. I made a start trying to disassemble the coil but failed at the first hurdle – can’t work out how to unplug the two electrical cable plugs from the top right… They seem stuck in and don’t just pull out. I don’t want to force them.

Thanks in advance.