I'm guessing if it is not 6 then I need an examiner to sign me off, so any previous examiner that I have flown with should be able to do this, by proxy, right?
Any UK examiner that was a six himself, could sign you off as a six. That was essentially the gist of the grandfathering scheme. But I have no idea whether this scheme is still alive today (and Whopities post kind-of suggests to me it isn't - but the CAA would be able to clarify that for you).
If the examiner can't sign you off anymore (maybe because the scheme has expired) or if you're not a six, you're due for a formal exam at a language institute. Expect somewhere between 100 and 160 euros for such an exam, and a duration of about 20 minutes.
(I seem to recall that there was one further limitation, but I'm not quite sure about it anymore. The examiner could sign you off, but only during some sort of flight test. So if you visited the examiner just to get his signature for a revalidation by experience, that visit was deemed to be too short for the examiner to make a proper assessment. But as I said, I'm not sure about this. I do recall that the wording on the regulation was rather vague about it though. In any case, *instructors* have never been able to sign you off, only *examiners* could do that.)