Here in the US, the authorzed ML repair center, not the dealer, handles repairs and it is up to tge repair center's discretion as to what gets replaced. If the dealers are not authorzed for repair, they would typically simply replace the unit for the purchaser and then seek compensation from ML. Do you know if your dealer runs a ML authorized repair shop? [I do not know how ML UK handles such things compaired to ML USA].
What you have been describing really sounds like a likely coil problem or a problem with the coil connector on the contro head. If they opt to replace both components, to be sure, you are basically getting a new detector so I would personally be OK with that. Since they cannot replicate the issue themselves, I would attempt to 1) get a completely new unit, 2) have the coil and control head replaced, 3) have only the coil replaced, and last choice 4) have only the control head replaced because it is least likely to be the issue.
Hope this information helps guide you with further questions with the shop personnel.