So in summary - the only real areas of discussion I should have with the fitter is:
1. Please remove the film. The manual warns users - that with all gas burners being used - the extractor could experience a significant amount of heat - and get extremely hot. It was supposed to be removed before final fitting. If there is warranty issue in the future - and Neff are called out - they may choose to invalidate the warranty (at their discretion) - if it looks like basic attention to detail was not payed. At minimum - it will be hard to explain that an appliance used above gas burners - and made of metal - should be partially covered in thin combustible plastic film anywhere.
2. Once the wooden panel covers the top - the plug (therefore isolation point) will no longer be directly accessible. As there are no RCD's on the main CU, an isolation strategy is necessary. Having to unscrew the panel - to access the isolation point - might not be enough.
3. A number of fellow community memebers have suggested that the power cord be hung on the cabinet on the side - just to protect it from heat.
So other than these points the job is mostly complete (inside the house).
These final improvements - will ensure that there are no issues from this install - that could cause a problem with my home insurance & the manufacturer's warranty.
My sincere gratitude to those that chose to constructively contribute to this thread.
I learned alot through the discussion - and I hope it will help others in the future.
Some notes for future readers:
The Alu-flex exhaust pipe - was sourced from IKEA. It is extremely high quality and good value for the price:
https://www.ikea.com/gb/en/search/?k=NYTTIG+TUB
Don't listen to anyone that tells you duct tape is appropriate or acceptable for a cooker hood / extractor seals. Use Aluminium/foil tape that is specifically rated for heating ventilation systems. It must be heat resistant.
Competent install - will allow for relatively easy access to the isolation point.