Had my battery replaced a year or so ago, had no idea at the time what current spec battery I had in and knew nothing about AH/CA etc. Anyway was replaced with a 68ah and 600CA battery.

Ive had a few rare intermittent problems with the car that dont generally last too long, throwing random errors on the dash that dissapear when you turn the car off and restart etc. One of those things could be a random sensor playing up however have read power dropping in the battery can also cause ecu issues.

Mechanic found for me min requirements for my car (07 FL2 petrol) 75ah, 700ca. Question is would my battery being slightly lower spec be concern and should I replace it first before starting to replace sensors or do you feel it should run the car ok regardless?

Run it like it is now. Don't worry, the battery will handle it on normal operations.
For your peace of mind, there is a starter fuse that's 150 Amps. So the starter, at its worst, won't draw more than 150 Amps. Normally much lower than that. And your battery being 600 Amps (CCA - cold cranking amps at -18 deg. C) will do just fine.