Quick update on this.
Eventually went to Power Tune in Motherwell for a remap. Joe did an excellent job, and explained everything very clearly before and afterwards. The cost was very reasonable, and a lot cheaper than some of the tuning kits I was looking at.
The car is transformed, better torque across the rev range, no flat spots now and mpg improved by 10-15%, depending on driving style!
All in all, just what I wanted. Cheers for all the advice, guys.