Well as promised an interim update on my Omega issues.
Firstly thanks to all who offered advice and solutions, it would appear as suggested by some that I have two concurrent problems. 1. A slight loss of coolant fluid. And 2. An intermittent miss-fire. As I finished work on-time today I popped down to see my mechanic. Whom I've used down the years for jobs I can't manage. Having described the symptoms to him, he connected a Snap On diagnostic box of tricks, he programmed the vehicle details, run a diagnostic check, cleared the resultant codes and run again. Results show I have a misfire on no 4 cylinder, so at first opportunity in daylight I will be checking coil / plug for condition and oil / water ingress. Just one thing to ask Sir Philbutt, am I mis-understanding your advice here "you shouldn't have to disconnect at the back as it's awkward if not used to doing it." Am I to disconnect it at the back or not? As for the water loss, I shall be monitoring all the usual suspect places ongoing.
Once again, much gratitude to you all for your advice.
JamesV6CDX, I'm located in Northamptonshire, although my job take me all over the UK, but not in the Omega!