Thank-you David and D2 for your responses.

That is good news to hear the injection control units are compatible.
It will be interesting to see if the problem is related to these computers.
Also will be interesting to see if there is any noticable performance change
between the two units.

The fuel pump relay was previously replaced with no change to the intermittent problem.
I have just swapped the ignition module and cleaned the grounds at the
ignition module. I have extended the duty cycle pin 3 to the inside of the
car to monitor it. It is probably worth swapping the OVP relay however
tapping and wiggling the relay and fuses does not cause the problem to
occur.

About the only clue that might have some significance is that this
apparently does not tend to occur on highway jaunts, which makes me
suspicious that at lower speeds and around town the extra heat in the engine
bay may be the catalyst for the intermittent component/connection.

I am also considering extending the fuel pump supply to a voltmeter in the
car to see if it disappears at the same time as the problem occurs.

My dad is going to use one of the cars for his winter driver , so will
get him to report back as we swap items.

I will report any progress or results......George