Yes, you will need to split the upper cam carrier away from the top of the head (it means removing the cam belt, so you will need a cam locking tool for refitting and setting the timing). The mating surfaces will need cleaning of all oil and remains of old sealant, and then apply a thin bead to the areas where the parts make contact (there is a diagram somewhere on this site I think, or you will find one in a Haynes manual).
You must use the correct type of anaerobic sealant - some sealant is likely to squeeze out into the oilways, and an anaerobic sealant will set between the mating surfaces where the air is excluded, but any that oozes into the oil ways will remain liquid and be flushed away by the flow of oil without causing any blockages. Using a normal type of sealant or RTV is very likely to cause blockages in the oilways, and may restrict or stop oil getting to the cam journals, the hydraulic lifters or to the valve stems which will cause future problems due to wear.
You can either use the genuine MG Rover sealant (part number LVV000108EVA), or Loctite 574 which is the commonly recommended alternative.