Ok, new development!! I had a look this morning and did the following -
Firstly I wiggled the wires and connectors on the offending side - no change.
Next I split the connectors Found some corrosion in the offending side clip and cleaned it up with a screwdriver and electrical cleaner. Re assembled and tested - no change.
I then swapped over the light units and the fault remained on the same side - so not the light fitting.
Next I tried swapping over the short light harness from one side to the other - now both sides don't indicate, even when I swap the harnesses back and forth... Grrrr!
The short light harnesses look identical so I doubt that swapping them has directly stopped the good one from working. The only possible conclusion I draw is that the fault is coming from the car and has broken both of the short light harnesses that have been plugged into that side as I notice that there are electrical components under the black heat shrink.
I'm fairly handy with electrical items, so happy to do some more testing and fault finding if someone can give me some pointers or electrical drawings (or both!

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Any help is welcomed!! Or else it's going to have to be another trip to Murrays
Thanks.
P.S - I tried unplugging everything and confirming the light units and short harnesses are back in their original places and all connections are good and something slightly different appeared to happen, the left indicator worked ok for about 3 clicks then stopped all together and the dash indicator flashed/clicked fast again. For completeness I also turned on the lights and confirmed that both rear LED rings operate, but only the right hand side LED ring flashes when I put the indicator on, the left just remains lit.