The fault sounds electrical from the mechanic's diagnosis.
Follow Simon's suggestion and change the relays, use good quality units , not cheap pattern parts.
I run a +8 , but I think a lots of the ignition system is similar to the Rover T16 engined +4.
The first thing I do when faced with an electrical issue is to check all the connections and especially those to earth, but an experienced mechanic will almost certainly have done this already.
If the relays do not cure the problem , I would look at the ignition module mounted on the left (looking forward) side valence. I have had to replace mine once. The other area could be the distributor and its associated modules. The general method of isolating this is to use another known good one to replace it , but I accept that could be tricky in Hong Kong
Morgans of this age do not have an OBDII port , but they do have a port that can be connected to a RoverCom system . However this was short-lived technology which gave way to OBDII . It is possible that a Land Rover garage in HK has an old one lying around somewhere? That may be worth a try.
