I'm sure this must be gearbox, but what to do?

[HPsauce]

I've been getting the odd "thump" for ages now (years) and it sort of feels like the box "might" be changing gear then not rather quickly.
It's usually been associated with steep hills and corners so I'd also vaguely wondered if it was ESP-related in some odd way.

Anyway it was a bit more noticeable yesterday (and I wanted to have another "play" with the dodgy cruise control too) so I decided to go for a little "test drive" today in the snooker mid-session interval.

The cruise control did work normally the first time I tried it (until I braked), after that it wouldn't engage any more.

Back to the box.................
I tried driving in manual mode for quite a while, so that the only "automatic" changes would be down due to kickdown or slowing down. This is a PF system so no separate kickdown switch, so the kickdown mechanism (whatever it is) "could" be part of the problem.
I couldn't get it to thump or judder consistently, but it did happen several times. No "precise" pattern, but the following observations from today and before:
1. It definitely happens in 3rd gear, and quite likely in 2nd too (not really there long enough with my foot down! or 1st!! ) but not in 4th or 5th.
2. Sometimes it's just one thump, others it a judder of maybe 3 or 4 max over maybe a second or a tad more.
3. Engine revs just seem to rise normally but there is the feeling of a brief loss of power. Again I have wondered if the ESP is doing something at times.
4. The problem appears "typically" (but not definitely) going up a steepish hill in 3rd gear at 3000 revs or less. More often on a bend, but they're often the steepest bits anyway.
5. Kickdown works normally when I floor the pedal in typical situations, including from 4 to 3 and 3 to 2.
6. It probably has spent 99% of it's life in top (occasionally 4th) gear, being used almost exclusively on long journeys; no commuting or shopping trips!

So, any thoughts anyone both on what it is and what to do?
e.g. Time for a gearbox fluid and filter change?
(it's done over 120,000 miles and had no work as far as I know)
And if so, is there anyone trustworthy not too far from me - I'm near M40/M25?