Ozymandias, thank you for that link to the CAA document.
My problem is that though native English speaker, I have a French licence, and all my flight tests are conducted in French and not with a native English speaker - so I will not be able to ask for level 6 upgrade at my next flight test.
But paragraph 2.1.1.3 is interesting - "Assessment of English language proficiency at level 6 need does (sic) not require assessment by an organisation or individual so qualified, and may be undertaken by another native English speaker".
I wonder if that means that I can just ring the CAA, ask to talk to licencing department, and request that my CAA FRTOL be amended to level 6? After all, you have to have good English just to get through the answering machine service!