An update on the problems.
Getting nowhere with the Mk1s because of very worn slides and at around £45 EACH not wanting to buy new just to try them out I refitted the Mikuni's, first problem one of the choke plunger parts was missing (1 into 2 cable chokes) so ordered a new one.
When this came I noticed it had a weaker spring than the existing one but other than that was right.
So setting these with the slides fully down and opening together I then screwed the throttle stops in equally until it would tick over nicely and took it for a spin up the road. within 200 yards it started missing on the front cylinder and this got progressively worse in the next 200 yards until it was only running on the rear cylinder.
I took the front float bowl off thinking it may be fuel starvation (I had turned the fuel off straight away to check this) but it was full.
I then started on the ignition for the front cylinder thinking something may be breaking down, so made and fitted a new copper cored HT lead (the exiting one was a decades old silicon car lead), try again and no change, next I changed the condenser, try again, no change, but this time when I turned round to come back there road was empty so opened it up more and it then picked back up on the front cylinder. this then led be to think it was possibly flooding the front carb (but not enough to overflow), or excessively rich, so decided the next day I would swap the carbs over.
Before I got round to this the I remembered the weaker choke spring mentioned earlier along with the fact I had changed from a Manx tank to a modified Dommi tank so I swapped the choke assemblies over and rerouted the cables so the junction box was below anywhere the tank could interfere with it.
SUCCESS, it now ran reasonably well, enough for me to know I had solved that issue, I've now balanced the carbs with vacuum gauges and it is almost right, I had no spare new plugs so ordered some and once I have made a new throttle cable for the front carb I will set about final tuning.
So it was either the weaker choke spring or/and the tank interfering with the cable junction box meaning the front cylinder choke was always on. None of this explains why I have always had trouble as I only swapped the tank and carbs over this winter, probably just my incompetence.
Chris