Dunno about the legal situation but I'm quite sure you would end up with a lot of explaining to do if they showed up in a customs search in many countries.
Why on earth do you need diamorphine, unless he's intending to practice on the side (which would also be frowned on)? Does one of you have advanced cancer, or are trying to sort a dependency problem? There are more acceptable strong pain-killers like dihydrocodeine that will deal with 99% of yachting emergencies.
My understanding is that it is perfectly proper to carry morphine for use in extreme pain - indeed it is recommended for long passages. You could consider Oramorph which, despite being morphine sulphate, is not actually treated as a Controlled Drug here in the UK - and has the benefit of not needing to be injected (which may be an issue if the Dr is also the patient!).
When I did my 'Ship's Capatains Medical Course' we were instructed in the use of morphine and also told the regulations about storing them on board. Must be in a locked cabinet secured to a major bulkhead or something like that. I know I have to sign for them when I take over some boats for ocean passages.
though i'm a medical ignoramus, isn't diamorphine just the pharmaceutical name for heroin? in parts of south east asia the penalty for importing it is death!
maybe you could also carry cocaine as a local anaesthetic and marijuana in case you contract MS?
i'd have thought it would be safer to stick to less controversial alternatives even if they might be a little less than ideal!
Your problem is that every country in the world has it's own legislation - Certainly Merchant ships carry dimorphine - in a bonded locker - Have a look at Ship's Captain's Medical Guide on the MCA CC site. Or the medical page on my web site for the link.
I know its excellent for when something high up the analgesic ladder is called for. It does make some people sick so an "anti-being sick" drug is often used at the same time. Anti-emetics or however you spell it!
In order to have MCA category 0 coding (commercial operations anywhere) you have to carry the full medical kit and this kit includes dimorphine. I was mate on a 50' boat in an Atlantic circuit (TallShips 2000) and we had dimorphine onboard. We bought the kit from a specialist supplier and just had to show evidence that the boat was category 0 coded. When I did the 'Ships Captains Medical Course' we were told that the certificate entitled you to take charge of the medical kit, and thus be responsible for the drugs.